Well to me it's really quite simple. Look at the facts- for one, they said they'd send supplies right? Well we know that there aren't any supplies except gold, books, and cloth- not food in that cave. Also, if this were their HQ, don't you think it'd be much more heavily guarded considering the loot? I bet you this is just an outpost.
Also, they were obviously not afraid to take on Caruthers, so they either have a massive trick up their sleeve, or they have an entire force to be reckoned with. If they weren't bandits, then why are they hiding in the first place? Also, they did mention 5% of the "business" right? I find it hard to believe that a functioning business would be run out of a cave with no supplies, a few sleeping bags, and a few chests of loot. You gotta think, how did these guys get such amazing treasure in the first place, right? Not some common folk, I tell you. They're hiding something, for sure, and Geran will intend to figure that out.