Misty Breez is self-custodial. You're referring to L-BTC, which is held in a Liquid federation. There are 15 functionaries (all from different entities) in Liquid, so no need to FUD in order to get ppl using your service.
Discussion
Unfortunately this "self-custodial" term is being widely abused by the biggest VC-funded companies, who used to be Lightning-native, and are now focused on private, proprietary networks, usually sitting behind fragile APIs run only by one or two companies. I understand there is money to be made, but it's not cool.
Again with the FUDing. Self-custodial is a regulatory term. If you hold your keys to the asset, the wallet is self-custodial, no matter ls what the asset is BTC, USDT or L-BTC. Now, the question is how protected the asset itself. With BTC, if you have a way to unilaterally exit to mainnet (which Liquid doesn't, but Spark for example does), you have control over your funds.