spark's claims of self custody are extremely questionable. they are pointing at the escape hatch and saying that it's good enough to call this self custody. try to read the documentation of how to exit from a statechain such as spark or arkade. if you have permission from the operator to go on the happy path, you can remove your funds by paying for a couple of fancy taproot transactions. they are not as cheap as a simple bitcoin payment, but maybe it won't burn everything you had. if you don't have their permission to leave on the happy path, you have to unwind all of the VTXOs you may have. this will cost a lot more. depending on the statechain implementation details and your own personal usage pattern, you may have to pay for many many transactions to unwind everything and claim it on L1. we know this trick. if the cost of unilateral exit is more than your stash, it's the same thing as using a custodian.