Indeed this is a bizarre criticism.

Why wouldn't signal market a crypto property that is especially important in the context of a messenger app?

Because lay users misunderstand "Perfect forward secrecy" as "perfect security". It's actively misleading when included in marketing material targeted at a lay audience.

OTOH it's also probably read by most as meaningless hyperbole, so the actual damage from being misleading is probably minimal.

I also hate the name, but its not like signal was the one who came up with the term. I somewhat think most marketing discussions of cryptography are highly misleading, although i suppose its reasonable to want signal to be better.