No, that's not correct. Any airside departing passenger at LCY can go to gate 2A and ask the staff at the BA service desk to let you out. This takes 20 seconds if you are HBO. You arrive after immigration but before customs.
(If you aren't flying BA, perhaps the BA staff member will want to get in touch with an agent of your airline - but I was flying BA to a non-CTA destination when the above happened to me a few days ago, and anyway nobody wanted to know where I was meant to be flying, nor did I need to show my BP or ID.)
It is impossible to access the airside departures area at LCY without being inside the UK. Therefore, all airside passengers are in the UK until their flight departs. (And for this reason you are not making any customs declaration when you exit airside in this way, as all your belongings were either in the UK already, or had just cleared customs.) Though if their flight is cancelled it is possible that some passengers' permission to be in the UK will have expired on that day and they need special authorisation to continue to stay in the UK.
A consequence of this is that LCY does not offer transit. If you are e.g. arriving from Schengen and departing to Schengen - you still need to enter the UK then go through security again - like a lady who I allowed in front of me at the LCY immigration queue last week because she said her onward flight was already boarding.
At LHR, LGW and MAN only, some passengers in the airside departures area are in the UK (or the CTA), and some are not in the UK. That is the reason those airports have the biometrics system, and LCY doesn't.
In theory if you start a domestic/CTA flight at these airports, your ID should not be checked by the government (airlines ask for ID to check bags or for revenue protection, and the police always have the power to ask for ID at ports anyway). Hence the biometrics is used to confirm that you were in the UK before entering airside at those airports.
If you arrive in LHR from the US and are transiting to Schengen, you don't go through UK immigration, while if you arrive from the US and are transiting to a domestic/CTA flight, you do go through UK immigration and biometrics. But both sets of passengers are able to mingle in the airside gate area afterwards. (Not entirely sure how things work at LGW or MAN.)