So in order to trigger this behaviour, someone with root-level privileges needs to edit a Unit file and enter a “invalid username”, in this case one that starts with a digit.

But you need root level privileges to edit the file in the first place and to reload systemd to make use of that Unit file.

Source: Giving perspective on systemd’s “usernames that start with digit get root privileges”-bug

It’s an obvious bug (at least on RHEL/CentOS 7), since a valid username does not get accepted by systemd so it triggers unexpected behaviour by launching services as root.

However, it isn’t as bad as it sounds and does not grant any username with a digit immediate root access.