If what was true?

"...absolutely could be the modern town square and would be great if it supported real freedom of speech, in my opinion."

My claim is that (in the US) if a platform allowed everything that was legal it wouldn't be popular.

I don't see how a comparison with 4chan is any evidence for that. There are also heavily regulated forums which are not popular. So clearly that's not the reason for whether or not one is going to be popular.

Per you: how regulated/censored a platform is doesn't have an effect on its popularity

However your first comment says "far better in my opinion for twitter..."

I didn't say it doesn't have an effect.

It's not difficult to understand the argument: 4chan is the largest unregulated platform, much smaller than the largest regulated platforms.

And it's a fallacy because there are many differences between them.

Statistics is not a fallacy

Non sequitur.