The line of thinking is that if Israel is subject to international courts/laws regarding genocide for its action, then China will be too. China's participation in judging Israel opens itself to the same judgement.
1. https://en.wikipedia.org/wiki/Uyghur_genocide
2. https://en.wikipedia.org/wiki/Territorial_disputes_in_the_So...
I don't see folks buying that, sorry. In international realpolitik you play the cards you have and if your rival opens themselves up for criticism you play it.
Rhetoric trumps logic in this one.