"Firstly, the UN expert there doesn't say a word about genocide."
No, but the 800 signatories, mostly professors of Genocide Studies, History etc of distinguished universities across the world (including, for example, Stamford) did in the linked letter posted above in the thread did.
"Thirdly, they say "MAY" and don't suggest definitely happening."
“Israel has already carried out mass ethnic cleansing of Palestinians under the fog of war,” the expert said. “Again, in the name of self-defence, Israel is seeking to justify what would amount to ethnic cleansing.
And regardless of if you think ethnic cleansing is illegal, promoting the idea it's somehow ok is very morally dubious.