They are touting an small airport and 40 minutes connections. Tonight, of all flights scheduled to arrive at least 40 minutes before AY99 departure only DUB (STA 23:05, ATA 23:10) and BRU (STA 22:45, ATA 23:15) flights are delayed beyond the 40 minute limit.
But you are right that the last shorthaul wave usually has a bit of delay. And it is very true that AY69 / AY99 seems to have a departure delay "built-in". It rarely departs on time. But I think this has not so much to do with late incomings as it has with generous blocktime, favourable winds and the mindset of Finnair operations. I've been on the plane quite a few times when boarding is complete but departure still is delayed.
Many times they wait because they don't want to be early at HKG. Airspace is congested and they rather not be circling. But many times it is about mindset.
No one cares about delays for this flight, as delays does not propagate.
Adding 2 more weeks of flights since last post only worsens the statistics. Average delay is now 67 minutes and only 47% of flights arrive less than 15 minutes delayed. (01-May-2018 to 30-Jun-2018)
It is so poor performance that flighstats graphics broke. They are showing 5 stars because they didn't expect a flight to exist with 0,0 GPA
