No, it seems you are a bit confused.
You are talking about autossh, which is a completely different third-party SSH client tool that you have to install separately (as the link you shared describes) to have persistent SSH client connections and has nothing to do with systemd other than that you can start it as a systemd service (like any other third-party service).
OP is talking about systemd-ssh-generator, which is described here by Lennart Poettering (author of systemd) as working exactly as OP described it.
Yeah, the key seems to be in the comments from one of the changes: https://github.com/jellyfin/jellyfin/commit/0581cd661021752e5063e338c718f211c8929310#diff-bcc2125e56d5738b4778802ac650ca47719845aeee582f3b5c9b46af82ea9979R1176-R1180
It seems there was the potential risk that insufficient validation could allow reading arbitrary server files, which indeed poses a security risk.
However, my understanding is that this could be exploited only by authenticated users with permission to add new media. Not like that’s a risk to ignore, but it’s not like it could be exploited by anyone on the Internet.