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...
China officially recognizes the state of Palestine.
The Isreali supreme court itself has determined that Gaza is not Isreali territory.
However, situations like this, in which rhetoric and de jure policy conflict with de facto reality, open one up to others taking the fiction at face value. And what do you do then? Can't deny it without causing other problems. So now this may be regarded as an international matter because Gaza "isn't part of Israel".
An Israeli court can say what it wants, but can't have it both ways.
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.
The supreme court has jurisdiction over actions taken by the Isreali government, regardless of where those actions take place.