Whatever else antisemitism means, it has to involve some element of antagonism to Jews.

People who are antisemitic are indeed likely be to hostile to Israel (though not always - there are antisemites who welcome the existence of Israel, and would like to see all Jews go there).

But it is perfectly possible to be strongly critical of Israel, and of those who support it uncritically, and to do so without any hint of antagonism to Jews. That is demonstrated by many examples of Jews who fall into this category. This even extends to those whose criticism extends to being opposed to the existence of the political entity of Israel, once again including Jews who hold this view. The same applies in the case of any other country. (For example, those who favour Scottish independence and Irish reunification are opposed to the existence of Great Britain and the United Kingdom, without any assumption that they are antagonistic to any inhabitants of the country.)

It is perfectly fair for supporters of Israel to argue that their views are right, and that those who disagree are wrongheaded, irrational or unjust. But it is not in any way fair or logical to assert that being critical of Israel or opposed to those who support Israel is in itself any kind of proof of antisemitism. Only if there are grounds for believing that the basis for such criticism and hostility arises from antagonism to Jews can that assertion be justified.

And those grounds appear to be completely absent in everything that I have ever seen about the claim to identify antisemitism within Labour. No matter who makes that claim, Mudcatter or Member of Parliament.