It means that if a pool goes down between any period of two consecutive L1 blocks being mined, users can still access their funds by connecting to some other pool and showing them their 1-of-N identity MuSig signature (signed with their own key of course - they don't have and don't need to know the pool operator's private key).
Right so how does this proof happen?
Does the user have to post on-chain (L1)?
It sounds like an interesting idea nonetheless. It makes sense that a decentralised L2 that allows user scaling will have to rely on some kind of off-chain voting system, because otherwise the problem seems intractable.