So ntfy Android app polls a ntfy service. (Which is more efficient than each app polling separately) What does Google's Android library do if not also poll? Does it run a server on the phone? Does it get special signals from cell towers that most apps don't have access to?
It also polls, it just does it as part of the OS. Power saving by the OS can often interfere with apps like nfty that want a long-running background process, especially if they haven’t been opened in a while.
From what you're saying, it doesn't sound like the OS-based polling would be more power efficient, as someone in this chain suggested, just more reliable. Though, I think you can just turn off battery optimization for the ntfy app, in which case I'd expect them to be on par.
If that's right, it seems like ntfy with battery optimization turned off is effectively a "real" push notification system.
