As a Jew, active on HN for years, I have not seen this to be the case at all.
I have seen sporatic posts that appeared biased or racist on both sides of the aisle...definitely not more or less than exists in the real world... and certainly not enough to generalize about YC at all.
We detached this subthread from https://news.ycombinator.com/item?id=12488743 and marked it off-topic.
That doesn't necessarily equate to antisemitism but doesn't exclude it either. Anti-Israel sentiment is often associated with strains of "social liberalism"[0] and related political positions, leading to anti-Israel comments being made in the context of policy discussion.
OTOH HN readers have shown keen interest in Israeli technological developments. At times praise for their successes has been evident, so obviously not all comments about Israel are negative. I've had the idea there are distinct subsets of HN users with different attitudes. Depending on the subject matter, one or another group is more inspired to add comments.
[0] https://en.wikipedia.org/wiki/Social_liberalism
Edit: added reference
Really makes you think, huh?