There is no free speech on private platforms.

If that is the case, then the post I was responding to is wrong when it says "We need to put a ceiling on how extreme that content can be." If Facebook is just a private platform, "we" have no right to dictate anything to them.

That's a pretty slanted interpretation of what I said, what I meant may be better spelled out as:

"Because social media platforms demand extreme optimizations in favor of advertisements and general engagement, social media platforms cannot also be a conduit for free speech or un-moderated content as this combination will naturally favor extremist content and deliberately steer users towards extremist views that favor such optimizations".