An interesting aspect is the relationship of the airport opening to the limited testing capacity.
So far as I can gather, Hong Kong only has the ability to carry out a few thousand tests per day, maybe something on the order of 5000 tests or so. If the airport were closed, there'd be about 1000 more tests available for those already in Hong Kong. If the situation were to deteriorate, the need for that extra testing capacity might be severe.
The limited testing capacity also offers a suggestion why Hong Kong hasn't even hinted at relaxation of the residents-only rule: the ability to test more than a few thousand arrivals per day just isn't there.