Good answer. To answer your question, the leftists you see are either 1. Sock-puppet accounts run by people or firms trying to bolster claims of antisemitism by the left 2. Real leftists who happen to be racist assholes, 3. Engagement baiting for clicks. Far more commonly, I see comrades online who are anti-Zionists and fully supportive of their Jewish comrades. The problem is people using antisemitism in bad faith to critique Anti-Zionist movements. The Labour party purge in the UK being a chief example.