Hard to say exactly what the cause was, as I don't know what username they used on lemm.ee (doesn't seem to be the same one that was used on Reddit), but it's most likely:

1. They did not verify their e-mail address
2. They did not agree to follow lemm.ee rules in the sign-up form

Confirming new accounts is handled by the admins, right?

So it would need an admin to pass by this post and look what went wrong in that users case

Good.