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.
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.