[bjterry]

That is not at all in conflict with the post. The selection effect is one of the sources of the empirical diseconomies of scale the author mentions.

[CityOfThrowaway]

My first read was that the author was claiming that smaller companies are at least as competent at moderation than big platforms.

In my first read, I took that to mean from a process and inputs viewpoint.

But reading it again, I suppose the author could be taking a more consequentialist point of view. They are at least as good (or much better), largely because they don't have to do much.

I suppose that's fair, but also a lot of words in the OP to state a truism :-)