Before making a case for accessory to genocide, they'd first need a binding judgement that it is a genocide, presumably. I agree it probably isn't as clear cut, or at least not as simple.
The problem is a decision it is plausible it is one isn't a ruling that it is one. There is no legal basis for deciding someone is an accessory just because it is plausible. Personally I consider it a genocide, and wish there was a legal basis for going after people as accessories, but there just isn't yet.
Have they prosecuted any of the actual state parties to the Rome statute who are still providing arms to Israel?
I don’t think it’s as clear cut as you say it is.