That would have been my guess as well, but apparently the Animal Crossing save is completely separate, and tied directly to the console. When you do a User/Save transfer, if there are no users left on the Source console, it creates a generic "User" account, that you can then use to access the Island Transfer tool and do an island transfer.Interesting. How would it transfer the island if the user transfer deletes the user? Wouldn't that then also delete the island save?
This is what I'm planning on doing during my lunch break, and I'm just going by the guide on Nintendo's support site as well as Nintendo Life's guide.
It definitely is confusing and not obvious without the online instructions.
Nintendo Support: How to Transfer Animal Crossing: New Horizons Save Data
en-americas-support.nintendo.com