They wouldn't be branded as antisemites because that's not in the definition. The only thing that comes close is the (correct) assertion that implicating all Jewish people with the actions of the Israeli government is antisemitic.So let's say Labour take the definition as is (despite those unworthy Jewish groups also being against it, for the reason that they feel it conflates legitimate criticism of Israel with anti-Semitism). And then someone in the Labour Party (likely on the left) makes a legitimate criticism of Israel. What will happen? The people in the party against Corbyn will start foaming at the mouth, calling for action against those members because "well, we as a party have accepted the definition as written! This IS anti-Semitism!" So then they have to turf out members for no good reason or, not take action against them and have the faux outrage warriors (who are motivated primarily by being anti-Corbyn) dominating the next media cycle for Corbyn's failure to combat anti-Semitism.
It's got nothing to do with whether they're Jewish or not and everything to do with what their organisation is there to do.I'll say it again, the wrong type of Jew. The only Jewish people allowed to be listened to are those who take any criticism of Israel as anti-Semitism, and anyone who contradicts them, even if they are Jewish, are anti-Semites.
Not everyone involved in the IHRA is Jewish, but everyone in the IHRA is there to fight antisemitism. It's not the being Jewish that matters (although that lends more credibility), it's being an organisation set up to fight anti-Semitism.