it signals to “normal progressives” that there’s a line

Well, calling someone a communist doesn’t always have the same negative connotations that it used to have. Because of that, they wanted another term that means the same thing but has negative connotations, which is why they picked up the term “tankie”, while disregarding the actual meaning of the term.

It’s really annoying, because the term was actually really useful, but is far less meaningful than it used to be.