Assuming they own the copyright (which I believe they do, since they were able to relicense it to begin with) they can absolutely offer it under a dual licensing arrangement even if the licenses are incompatible. It would only be an issue if other peoples' AGPLv3 licensed code was in there, but as it is not the only copyright they would theoretically be violating is their own, which is literally not possible.
Dual licensing under a free software license and proprietary EULA is a common business model, especially when the free software license is a strong copyleft like the AGPL, since the proprietary licensors do not have to abide by certain conditions that free license users have to.