How do independent servers solve the problem of scaling moderation? Every video still has to get reviewed by somebody, so the total number of reviews done doesn't change.

No single company must pay for the army of reviewers. Different servers compete for good content, i.e. have incentive for good moderation at low scale, which is possible.

Okay, that's the theory, but in practice, how good are Peertube servers at, say, filtering out djihadist beheading videos?

EDIT: A quick search shows that I was asking the wrong question; in practice, there are servers dedicated to exclusively these kinds of videos.