Hi Mathias, although it helps to know that Principals can be an umbrella term for canisters or users, I can understand the confusion. I think the documentation intentionally uses “Principal” to indicate both users and canisters.
If you read further down for cycles wallet docs it says:
‘A user principal or canister principal can be assigned to a controller or custodian role.’
Although lengths of principals in user vs canister is different, the cycles wallet will accept both types of principals as a custodian or a controller.
Also, there are some updates to the cycles wallet as described here: Future of the Cycles Wallet - #10 by kpeacock