No, the argument for "even federated ecash is custodial" would not be dependent on that. It's an argument to be sure, there is more than one facet here, but the power that the (federation) mint has over you is the issue, and that doesn't disappear because what their token represents is not bitcoin.

(In the extreme, if they didn't claim their token represented anything, not even trust in their integrity, then the token is meaningless. If the token is supposed to represent value, then there is a trust issue, so that "self-custodial" is a problematic description).