It's actually my opinion. I'm not claiming it is a provable fact. It is just what I think might happen in next 50 years let's say.

HK has been independent for a long time and been given back to China quite recently so I think 100 years from now if it stays as part of China it would be much less independent.

For example, take a look at Kaliningrad which is completely Russian now. Or other parts of Europe which were assimilated when taken over by bigger nations/states.

Or you could look at ex-Yugoslavian states, Catalonia, Scotland. Kaliningrad is one Russian example, but you also have the ~14 countries that used to be part of the USSR and chose independence. Things can swing either way, even after prolonged periods of integration. For HK it will depend on how much of a separate "Hong Kong" identity can survive.

Yes, and the former Soviet republics have all either remained in the Russian sphere of influence or have moved aggressively into the Western one. Either way, the simplicity of this “choosing independence” idea conceals a considerably more complicated and delicate reality that more closely resembles the grandparent poster’s argument. :-)

This is fascinating: https://en.m.wikipedia.org/wiki/Kaliningrad

Though in Kaliningrad's case the original inhabitants were evicted.

Yes. Also see: https://en.m.wikipedia.org/wiki/Kaliningrad_question

It's completely hypothetical but there are some people arguing it should be returned to Germany as it used to be German for many centuries before.

It’s very possible. Especially if the HKers move away. Funny, the doom and gloom predicted for 1997 did come eventually, just 20 years later.

How was HK independent when their leaders were directly appointed by the British government?

I think, in context, the meaning was that HK was, for a long time, independent of China.