I suspect they’re referring to the 99 year lease the UK signed in 1898 with the Qing dynasty. [https://en.m.wikipedia.org/wiki/Sino-British_Joint_Declarati...]

As to how they think that has anything to do with their points, it doesn’t of course - and the UK agreed, which is why they left. Also, because it’s not like the UK had any other choice.

I’m referring to the start of the agreement, not the end.

Fair, but why does that matter? The UK voluntarily relinquished control and handed HK back to the PRC

It did and I don’t think it could have gone to anyone else, leaving the choice as giving it to China, or keeping it.

The argument that it shouldn’t have gone to the CCP was one I heard from someone who lived there.

Why not? The PRC is the successor state. It makes less sense to hand HK over to ROC because the ROC never had sovereignty over HK.

Successor state by force, not by people’s choice. The small part that ended up democratic hasn’t chosen to join the PRC.

Not only were the PLA and CCP massively popular because of the insane corruption and hyperinflation under the Nationalists, the ROC imposed (at the time), the longest martial law in human history [1]

Taiwan is democratic today, because of transitional justice, but at the time when the PRC succeeded the ROC in China, the nationalists led by Chiang were as dictatorial as you can get

[1] - https://en.wikipedia.org/wiki/White_Terror_(Taiwan)