Numbering hidden services is something you publish papers on, and most researchers arrive at much larger numbers than 4000. But just as the author pulled numbers out of thin air (20 out the 4000 are deemed worthy of anonymity), 9 out of 10 paedophiles caught by him would turn out to be other researchers (or "researchers"). They are much more likely to run the spyware he offered.
At any time, Tor is frequented by many like him. Some law enforcement, some working for child abuse organizations, some academia, and a lot of regular people with the same ambitions. Advertising a high child porn site is the best way to attract them. Most likely many of them have an unhealthy interest in this stuff, and line between "researcher" and paedophile is not always clear. The law does not make a distinction.