Gonna have to push back here for a few reasons: 1. Just because a historian is conservative or even anticommunist, does not mean they are liars and their writings, if skewed toward their own ideological preferences and those of their audience, isn't based in fact. 2. I found A History if the Jews on Libgen, and the source that he cites is Howard Sachar, ‘The Arab-Israel Issue in the Light of the Cold War’, (Washington DC), 1966, 2.
I don't know about Howard Sachar's attitude toward communism, and I couldn't find a copy of the book that was cited, but I did find Dreamland: Europeans and Jews in the Aftermath of the Great War which contains about 150 pages about Rosa Luxemburg that casts her in a very good light from what I can tell. So not a guy with an axe to grind against communism.
Unfortunately I can't find an independent resource supporting the original claim (although I swear I've seen one somewhere) but your argument is, at best, disingenuous and based on vibes