What kind of public api has access to the full user list?

It’s the API that ALLOWED the misuse in the first place, so the developers are the ones to hold accountable.

I'd call this API misdesign instead

This isn't surprising, a user found out how to abuse the API to progress without doing lessons by just submitting the expected parameters last year. If they followed through on reporting to Duo then they've had plenty of time to address it. Seems like the mod also found out how vulnerable Duo was and took it to it's natural conclusion.

Crap, I can't remember what email I used or what name was on it

I used my spam email and my name on there is a meme so I'm not really worried.

Well, I use a fake name and an email used only for Duolingo, so..... ok.