He's saying he thinks that there would be 170k criminals forming a singular network seems like a high number.
To me, it sounds low. There are very niche interest websites with 750k accounts[1]. Further, 170k isn't the number of criminals, but the number of people who had interaction with Sky - Which may be basically anyone who had a shady friend.
The problem with our modern understanding of "innocent until proven guilty" is that unsubstantiated rumor can spark fire very quickly, while real damage can fly under the radar for a very long time. People who have been actually damaged often try to extricate themselves from the situation as quickly as possible, or end up trapped and pushing further into it. Without real numbers to back this up: If you marry an abusive spouse, Either you get a quick if painful divorce, or you normalize it and become embedded deeply in the relationship and cut off from your friends who tell you to get away, because their advice for setting boundaries only gets you hurt further.
I have no doubt that the niche interest of shady behavior can attract a few hundred thousand users, while staying unknown to anyone outside the niche.
[1] Furaffinity claims this number, as a standard internet oddity, but fimfiction.net boasts nearly 300k; Many other "weird" websites probably have numbers in the 50-500k range.