Based on my read of the `minecraft-server` package [0], I think it is using a management script wrapper around the raw Minecraft JAR [1], which implements the aforementioned idle functionality [2].

[0]: https://aur.archlinux.org/cgit/aur.git/about/?h=minecraft-se...

[1]: https://github.com/Edenhofer/minecraft-server

[2]: https://github.com/Edenhofer/minecraft-server/blob/421ff0ff3...