I mean if all the candidates are only from big companies or went to the same school or are all white men (this part I only figure out on videocalls), then I assume the recruiter isn’t fishing very many recruiting pools. I want recruiters who do more than search LinkedIn. They should be tapped in to local technology groups, alumni associations, professional groups, etc.

I don't know if this is what they meant, but I assumed they were getting at the idea that a good recruiter will find you candidates who are better than they appear on their CV (because they'll be cheaper relative to their ability), and it would also show they weren't just spamming for a match to the template.