I mean, I wouldn't regard it as a filter simply because any self-proclaimed leftist who supported Israeli genocide of Palestine would evoke genuine surprise from me. It's just not something you see very often. This is more a rule of thumb for common disappointing scenarios.
In response to Ben-Gurion's 1938 quote that "politically we are the aggressors and they [the Palestinians] defend themselves", Israeli historian Benny Morris says, "Ben-Gurion, of course, was right. Zionism was a colonizing and expansionist ideology and movement", and that "Zionist ideology and practice were necessarily and elementally expansionist." Morris describes the Zionist goal of establishing a Jewish state in Palestine as necessarily displacing and dispossessing the Arab population