Facebook, YouTube, Twitter, Instagram, etc, etc, all struggle with the issues of content moderation. The perfect example is Facebook pulling down a journalist photo of, if memory serves, a naked girl in a warzone as CP when obviously it was not.

Yup, these systems are imperfect.

But I think it's pretty silly to claim they're completely worthless or ineffective and therefore moderation simply cannot be done, and Parler is therefore off the hook.

Heck, HN has commenting rules/guidelines. Are you suggesting those shouldn't exist and any system to enforce them is pointless?