It doesn't run Debian, it runs (a fork of) Droidian which relies on Android layer underneath. There are other Linux phones that do actually run Debian and don't rely on Android.
I don't know what they are referring to, I can't speak for them, but...
• postmarketOS is unrelated to Debian.
• postmarketOS is based on Alpine.
• FuriLabs does not use Alpine or postmarketOS.
• FuriOS is Droidian is a Debian derivative.
• FuriOS is Debian running on an Android kernel, with Android in a container you can stop and start on demand.
Nothing you claimed applies here.
[0]: https://pureos.net/