Why would you believe that someone has compromised thousands of your users without evidence? It's the sort of thing you'd naturally expect someone to lie about. Furthermore, if such were the case, it would be egregiously stupid of Ulbricht not to change the way he protects user identities, and there's no indication he did that either.

After all, if friendlychemist did in fact have information on thousands of drug dealers, he ought to be able to get a pretty good asking price from the government, so why would he even contact Ulbricht in this scenario?