The notion is this: although taking over 50% of the Bitcoin hashrate would be a practical impossibility for any private actor or even a small country, China, as a country, has at times hosted over 50% of Bitcoin's hashrate, and it's thus plausible they could make this happen again if they wanted to.
I don't know why I feel repulsed whenever I see a mention of China in relation to cryptocurrency, anywhere. I guess it has to do with how China let this industry down by banning crypto activities in that country in 2017. That action was one that hit me hard at the time as a newbie in this industry. Then China was in a lead position in the space with lots of mining hash from there. That was then. In today's reality I don't think anyone should as little as accord China a second thought once this industry is concerned.
It's still legal to hold and trade Bitcoin in China. All they have done is precluded their banks from holding Bitcoin on their balance sheets.
China still contains a sizable percentage of worldwide Bitcoin mining, so it is by no means "banned" there.
And even if China hated Bitcoin, that doesn't mean they wouldn't attempt to take it over.