If Reddit were to reach out privately to this group, the first thing they'd probably do is ask for proof. It's trivially easy to provide proof you've carried out a hack; you just present some specific information that was not public and describe what all else you have in specific enough terms they know you're not bluffing. (Or, I suppose you could just send them your whole dump if you really want to make it clear what all you have). The only way the rest of us will be able to validate these claims is if they leak and it either matches users' own private account info or Reddit issues a disclosure about the hack (which I'm pretty sure they're supposed to do regardless).