I did try to create a new profile with the same email address and it didn't let me, so it does appear to be treating it sort of like a primary key.
And if it's just a label, then maybe it should be advertised as such. I see what you're saying now, but that's not obvious from the app.
After you changed you can get into the same account using your 2nd profile only. Ignore the email: site owners are free to confirm it, but playground does not.