On the defamation part of it, how about looking at it this way:

The anonymous poster (as I understand it) does not pretend to have any fact-based, special knowledge of Ms. Greenbaum. He or she is not saying "I know, as fact, that Ms. Greenbaum once poked Jewish kids in the eyes with sticks, whenever she had an opportunity," or "I know that Ms. Greenbaum was a member of a Nazi-worshipping club, and here is some of their literature." Mr. Anon is saying (as I understand it) that the position Ms. Greenbaum has taken on this issue of public concern is a position that a bigoted anti-Semite would take. Maybe he is even saying "Ms. Greenbaum's positions on a string of these public issues have demonstrated a track record indicative of bigotry and anti-Semitism." In either case, the anon poster is STILL talking about *issues* of public concern, even if presented as a "character" statement.   That's similar to why I think you could call the President of the US a "baby-killer" for instigating the hostilities in Iraq. The speaker's discernable intent behind using the words could be argued in various ways, but in reasonable application, they are addressing policy issues, not a "true" or "false" assertion that GW Bush has ever killed or been responsible for killing a baby.

(It's also possible - I really don't know - that anti-Semitism issues in policy, law and general public discourse are more sensitive in Europe and the UK than they are in the States, so that the way I see it is not the same way it would be seen on the right side of the pond.)