That’s what it says. From the same link as above, just below Step 1:

Important: You must use a personal Microsoft account with Windows Backup. Work or school Microsoft accounts won’t work.

I think it’s because to restore you MUST sign in with the same account you backed up with; if you drop out or get fired, you probably won’t have access to that account again. They may also be throwing back any email address with .edu, though that seems a step too far. I haven’t tried, that’s just my guess: they don’t want angry users screaming at them when they can’t get their shit back down from the cloud.