Burg's comments put Schultz's ability to support pro-Israeli policies at risk; if he's known to be an ardent supporter of Israel for ideological reasons (to the exclusion of concern for Germany), people will be more critical of his motivations when it comes time to e.g. send military aid to Israel.
Calling out your supporters, when being a supporter is politically contested, isn't a good idea. If Burg was lying, he'd be better off saying that Schulz doesn't give a shit about Israel.
Stating that Germany only exists for the sake of Israel is controversial, however. (At least, I hope it is! I hope most Germans have enough self-interest to dislike the idea of being a vassal state.)
Calling out your supporters, when being a supporter is politically contested, isn't a good idea. If Burg was lying, he'd be better off saying that Schulz doesn't give a shit about Israel.