They could, and this is definitely a RCE (a Remote Code Execution) vulnerability. GP confusion stems from the fact, that you (the forgejo user) must execute this attack on "your" instance. But of course the problem is that forgejo user can, in fact, be malicious, and use this vulnerability to escalate their priviliges from user to server.

It can't be done without authenticating first, but there's nothing about RCE that says that it must be sent from unauthenticated connection.