An update on some additional testing:
1) on appleid.apple.com:
I have tried using sign in in by adding a @me.com @mac.com and @icloud.com, all without without success
2) on iforgot.apple.com:
I tried resetting the account using any of the 3 above suffixes results in an error "<username> is not an AppleID"
3) on the iPad:
I tried signing into iTunes account unsuccessfully with a "Could not sign in" error
4) on the Mac:
I was unable to add this account as an iCloud account, but I could add it as a Game center account (System Settings > Internet Accounts > Other Accounts > Game Center
Selecting the account I can edit.. and edit the Nickname, password, secret question and location, but no option to add an email address...
In comparison to my email based iCloud account, you can only set the nick name or click on the button that says "manage apple ID..." which takes you to appleid.apple.com in Safari
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So it would seem the account is definitely not an iCloud account, but a Game Center account only.
I'd like to merge this with a valid iCloud account so they don't lose achievements / and saved games / friend lists