

why can they just sue someone for refusing to buy something
From the article:
As part of the agreements, the Chinese fund granted the remaining minority shareholders an option to require it to purchase their remaining shares for about $9.5 million, now valued at roughly $11 million, by early December 2025.
especially when buying it would be illegal
Legality in country A is completely irrelevant for suing someone in country B.
Also, in many countries having a realistic chance of winning the case is not a prerequisite to suing anyone for anything.

Cambridge Analytica was not enough of a reason to leave Facebook for me, because at that point I had already stopped actively using the site a long time ago and deleting my effectively inactive account seemed like a lot of effort for very little practical purpose.
This article (or just the headline to be honest) was finally enough to push me to actually go through with logging in one last time to finally get rid of it.