(a) First of all, most people don't know much about most things, including but not limited to antisemitism.
(b) Second, in the American context, I suspect the professional development of DEI staffers tends to concentrate, for understandable reasons, on race and sex, with comparatively less (albeit not zero) attention paid to other potential axes of marginalization (such as religion, disability, indigenous status, and class). I also think that people wildly overestimate the breadth and depth of knowledge DEI professionals have -- which is not a knock on them, they have a hard job! -- in assuming that any gap in their understanding can only be a matter of willful ignorance (and that every other group is the beneficiary of their infinite fount of wisdom and energy).
(c) Finally, while some "generic" principles of DEI training might be cross-applicable to handling instances of antisemitism, I tend to view antisemitism and other forms of marginalization as sufficiently distinct such that one cannot simply deduce proper orientation to one via knowledge of another, and so it is not the case that one knows how to treat antisemitism by taking what one knows about racism and cross-applying.