OP is not starting from HK and trying to get into the lounge with no intention to fly.
Bona fide is "in good faith without fraud or deceit"? OP entered the secure area under
5.1.B any bona fide air passenger awaiting an onward flight in any area and OP did want to fly, so he is "bona fide".
If 5.1.B does not have "awaiting" and retained 5.1.A "in the course of passage" then sure, it is not allowed, but so is going to lounge ex-HKIA not allowed.

Where in the legislation does it say OP can not leave the airport anyway? At most, it may be argued HKIA denies OP entry into departures 12hours before the flight as that is not a "bona fide" way of waiting for a flight, but once in, where does it say they can't leave? By forcing OP to cancel the ticket, the airport is forcing OP to become a non bona fide passenger, which they then can remove him, this is backwards logic.

I think the premise of the legislation and airport by-laws is that we are talking about a restricted area and passengers generally should not enter the departure level unless at the time of entry they intend to leave on a flight. Leaving the departure area is tantamount to entering Hong Kong, so the system is designed to discourage people coming and going freely even if they have a boarding pass. It is also a security issue if people are allowed on the departure level if they do not intend to leave on a flight. I can't speculate on OP's or anyone else's state of mind or intention when they enter. OP didn't complain and is merely giving a warning that you could get stuck. On my part, I am merely pointing out there is a legal basis for not being allowed to enter Hong Kong from the departure area.

They will let you out/back of course but it is made deliberately difficult and time consuming to discourage this type of activity.

What legal basis? Being let from departures to immigration is not the same as being let into HK, immigration still has to decide whether to land the pax, and they can deny entry for whatever reason, but that is an immigrant decision not airport. I'll agree to a transit pax not being able to leave if you agree a pax departing from HK can not use the lounge (must head directly to the aircraft.

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It's a possible argument. If someone enters the departure area and does not have the intention to leave the departure area on a flight, there is an argument that they are not a bona fide passenger. So if they insist on leaving and say that they intended to freshen up in the lounge and then go for a city excursion, they may be admitting they are not a bona fide passenger. It is just an argument and whether or not they have committed an offence would depend on the circumstances.

I dug around a bit and there is another legal basis for preventing someone from leaving under the Airport Authority Bylaw (https://www.elegislation.gov.hk/hk/cap483A). If they are in the "Bylaw Area" (which includes all the Restricted Area), then it is a condition of admission that that they are subject to the Bylaw and Terms of Use (which is basically any terms and conditions issued from time to time by the Airport Authority). I do not know the current "Terms of Use" but generally the Airport Authority has wide authority to regulate the Bylaw Area. Anyone in the Bylaw Area is also required to comply with all directions and instructions of an Aurorised Person or an Authorised Officer for the purposes of safe and secure operations of the airport. They are also required to observe and comply with all signs, notices and directions (etc.) prohibiting or restricting access to any part of any area, building or facility.

In the departure level, there are signs on doors saying that access is restricted so a passenger cannot just leave.

With tens of thousands of passengers per day using the airport and having their progress through the airport tracked (from scanning boarding passes at entry to departure level to leaving on a flight), it would be unworkable if people were allowed freely to leave, the the legislation and Bylaw supports this.

I am not saying anyone is right or wrong here - I am just setting out that there is a legal basis for restricting people from leaving, as this topic comes up from time to time.

Very interesting discussion, yet I have a very specific question that I'm hoping someone can help me with coming from the US Landing in Hong Kong at 8:00 p.m. with a onward connection at 8:00 a.m.. Without needing to go to any Lounge, am I allowed to exit the airport spend the night in the city and return back in the morning?

Why not, if you hold a passport that allows you to enter HK?

As mentioned above, sure. In fact I'd strongly recommend it. The airport is open 24x7 but the lounges are not, plus there's the noise and distraction of cleaners etc even if you can find somewhere to kip. Maybe going all the way to the city is not your best bet for a hotel, compared to something closer to the airport, I've stayed at the Silveri MGallery a few times and it's great, while The Regal is attached to the terminal itself, no shuttle bus or anything needed.

Of course, the city is so much more fun, 8pm arrival at HKG means with the Airport Express you can be in the city by 9pm and at your hotel maybe 9.30pm, grab dinner and soak up a bit of the vibe and then crash, but 8am departure the next day would mean being at the airport 6am to take advantage of a lounge visit, so that's quite an early start compared to an airport-adjacent hotel.

HEL FRA

yes - its going to the departures that causes some of the problem, sometimes.

ive had some luggage issue one time and FRA was very dissatisfied on me going in n out but didnt say i cant do it.

i eventually flew on that same ticket on that day.

hard to say if op has no intention to fly before being forced to cancel the flight. esp if he has 12 or so hours when requesting to leave departure airside.

and lounge staff did not send op thru to get arrested anyway

Of course you can! if you just want to sleep, book a hotel at the airport (the Regal is attache by a short walkway). Otherwise you can go into town on the airport express very fast (about 20 minutes) and enjoy a little bit of HK. Ideally leave your baggage checked through and just take what you need for a night as hand luggage - thus no need to wait for luggage or check back in again next day. Plan to get to the airport an hour or two before your flight. Security and immigration are a bit longer for non-HK people - but nothing compared to the USA.

Security is the same for everyone.
Immigration exiting HK is very fast.

I find it depends on the arrival time – fortunately I have access to the automated e-Visitor lanes but there've been times I have landed at HKG and the manual processing line for immigration has been chockers, so it'd really depend on what flights are arriving at the same time and where they're coming from. Rather like SIN, come to think of it – arrival areas seem to be either near-empty and people going through very quickly, or jam-packed.