I have an Idea what it could be. I'm not sure if the vulnerability has been fully disclosed yet. But what they could do is use the seed to regenerate the messages, check on which of these signatures have been issued, unblind them, check which of them have been spent, and then refund the delta. Obviously this is a lot of manual effort, but I think it might make sense for mint operators to have some kind of a fallback restore tool that automates these kind of things. It needs to be built first though. I've started a discussion in the cashu R+D chat