Coinbase has custody because they have unilateral access
In a fedimint no one has unilateral access to the funds
The group is someone
That someone has unilateral access to the funds
Therefore it is custodial
On a related note, multisigs sometimes take money from people, especially when a court orders them to. Case in point:
If the members of an ethereum multisig can receive and comply with a court order, why can't the members of a fedimint multisig do the same?
If they can do the same (and I'm pretty sure they can), how is that not an example of group custody (I'm pretty sure it is)?