You're conflating the inability of leftists to correctly guage liberalism's staying power (an ongoing problem) with describing liberalism's moral valence. Given the fact that liberalism literally stands atop millions of actual corpses, I think they're a bit closer to the mark on the latter.

Considering that every political philosophy has its own literal pile of corpes, is it useful to point out that liberal political systems have also waged war & persecuted people? It certainly seems a preferable ideology to the various authoritarian ones we’ve otherwise seen throughout history.

What system of government doesn't stand atop millions of corpses?