Even if we accept their username definition, when a problem is detected (User= fails validation), the system shouldn't run the service as root (ignoring the use

Even if we accept their username definition, when a problem is detected (User= fails validation), the system shouldn't run the service as root (ignoring the user's clear intention to run as some specific UID). The problem isn't the format of the usernames, it's that they detected an error, then ignored that the error happened while doing the opposite of what the user wanted. Failing the unit with an error would have been fine. Alternatively, proceeding with the indicated setuid(2) while warning about an invalid username would is also a sane response.