The thing not being closed as "not a bug" before the situation is figured out (e.g. the opener had a user account starting with a 0, so it could be expected input) or clearer context is given (e.g. if systemd had an established policy on what a username is allowed to be, a reference to that)?

While understandable (maintainers are busy people and likely have to deal with a lot of reports that have clear cut answers), it sadly fits into the image people have of them.

I personally think "thumbs down" (since you referenced those) is a completely valid response to that – but to be clear, posting insults or other attacks is not!