If the wallet is not custodial it means you own the keys, so even when you receive lightning payment the swap is mandatory, i searched on their documentation but i didn't find exactly how it works
Discussion
Yes, Muun is self-custodial. All your funds are always stored on-chain layer 1. As soon as you receive a lightning payment, the funds are credited to you on-chain through a submarine swap. Not sure how they do it when you receive a lightning payment, but your funds are never stored on layer 2. That would be custodial because it would use their node.