The rule of thumb is “remove all accounts” so if ATT appears to have some sort of prior claim on the device, that would be bad.
I definitely think it is worth further research how to remove them, because it is not clear ATT is claiming an account, as owner, is it?
Anything that exists in accounts, would be a blocker from what I understand.
It would trigger Factory Reset Protection. I cannot remember if you tried, but what happens when you do a Factory Reset from within Settings >> System ?
You might look at the Download mode screen, any clues there, you could perhaps send an image if in doubt.