Critical thinking is, however, not blindly accepting one's own thoughts and rendering them immune from challenge. It is anticipating legitimate challenges.

You have not anticipated any of mine, or if you have, you certainly haven't shared them.

I'm well aware of the free speech / anti-Vietnam War movement in the 60s, and the radical's imputation of malign motives to liberals (sometimes justified, sometimes not). Perlman doesn't add anything particularly to my understanding (civil rights leaders made very similar charges earlier), except a display of what it means to fail to engage in critical thinking and instead choose advocacy. (Why do I say this? Maybe you can figure it out!)

I strongly support free speech in large part because it's good to get ill-considered ideas into the open where they can be discussed instead of keeping them as private unchallenged dogma. "That was a dumb thing to say" does not imply "so you shouldn't have been allowed to say it".

Anyway, how about it? What is a good argument against, "no amount of placating imperialist powers will make them believe colonized people are human, and affirming that colonized people are dying for their resistance is continuing their fight" in the context of these protests?

Again, I can think of three extremely important ways in which this statement is itself inadequate as a justification for the sign. Can you think of any? You can rebut the idea if you want, too, but it seems to me so far that one cannot distinguish whether it's "critical thinking" or "reciting dogma".