I know a fair number of leftists of both Israeli and Palestinian extraction, and I don't really think this is true. The more nuanced and IMO correct appreciation of left-right politics in Israel (and MENA more generally) is that they're flavored but not inherently dominated by ethnonationalist movements that reached their fever pitch in the 20th century, and have slowly been replaced by ethoreligious movements that have substituted declining follower numbers for more extreme activity.