If anything, this would be an appeal to authority. It's not like the post is saying half of the American people believe it's genocide and therefore it is. No, the claim is that several relevant organizations like the ICC, ICJ, and Human Rights Watch, etc., are saying that it is genocide. That'd be like claiming that the vast majority of climate scientists believing in global warming is supporting evidence that global warming is likely true. It is. It's not enough evidence on its own, but it is evidence nonetheless.

That's the thing. Not all appeals to authority are fallacious. Supporting a claim with an expert's opinion is a logically sound way to support an argument.