One of the more offensive things here in this whole thing is that we're being asked to accept the premise that X is the most antisemitic platform there is.

No, you're not.

Advertisers aren't making their decisions based on whether X users are, or are not, more antisemitic than those of other platforms. They're reacting to three things:

1) The owner and de facto CEO is personally posting his agreement with anti-semitic tropes; and
2) The site's mechanisms to prevent major brand ads being shown next to toxic content can't safeguard that from happening; and
3) The owner and de facto CEO has said he doesn't care about, and won't address, those concerns.

It's not a boycott, and it's not based on X being "the most antisemitic platform there is." Advertisers are simply reacting to Musk's unwillingness to do basic things to protect their brands. Like not personally posting agreement with antisemitic content (to 100+ million followers), or to actually remove serial violators of their "Hateful Conduct" rules rather than ineffectually try to cordon them off. They don't really care whether there exist some specific ways to measure anti-semitism that might favor Twitter vs. other platforms in general - they want the "CEO" to take their brand safety seriously and keep their specific companies protected.

Musk is trying to reframe this away from these issues on to more favorable ground. But X isn't being "targeted" because of any generalized level of antisemitism on the site. X is telling their paying customers to f- off, and so they are.