I think the argument goes:

1. Israel is innocent of genocide (of course this is the standpoint of a lawyer defending Israel against accusations of genocide).
2. If the court decides against Israel, it will make provisions which will make it more difficult for Israel to freely execute its military strategies against Hamas (because the argument is that all of the military operations so far have had the sole objective of wiping out Hamas)
3. South Africa is therefore attempting to make it harder for Israel to pursue Hamas
4. South Africa is assisting Hamas, indirectly.

I think that's right?
So there are a few problems here, firstly the claim that South Africa is the legal arm of Hamas is clearly propagandising. It attempts to paint South Africa and Hamas as collaborators without evidence and it is a stretch to say this from the logic above.
Secondly, there is a fallacy present, it seems to me, in the assumption that if Israel were to be found guilty of genocide, then that would be aiding Hamas, which is unacceptable. This is a fundamentally flawed assumption: censuring Israel for genocide is a goal in itself regardless the consequences; crimes cannot be allowed even if they are perpetrated in pursuit of the goal of stopping other crime; Israel should be able to pursue Hamas without committing genocide.
It's also an unsound tactic because it does fit so well with the narrative that Israel blames Hamas for everything. When interrogated about questionable Israeli military actions, on many occasions, their representatives have publicly blamed Hamas, often to the point of absurdity. This argument therefore seems like an extension of that tactic.

That this is his chosen, and presumably best available strategy belies the shakiness of the ground he is on, and does not bode well for Israel's defence. The consensus among impartial academics is hat Israel is guilty of this crime, or is imperceptibly close to it.

It'll be interesting to see how things unfold, and I stand ready to have my mind changed from my current interpretation of the facts on the ground and the legal definition of genocide which are pointing to Israel's being guilty.