Flatpak?
And some level of immutability?

It doesn't seem like much of a step for Linux distros to cover the "gap" if/when we get any sort of viable mobile options for eg 1% of the market.

"uses regular Linux security mechanisms" is true regardless of whether any distributions you use configure them the same way or not.

The Android platform takes advantage of the Linux user-based protection to identify and isolate app resources.

As part of the Android security model, Android uses Security-Enhanced Linux (SELinux) to enforce mandatory access control (MAC) over all processes, even processes running with root or superuser privileges (Linux capabilities).

https://source.android.com/docs/security/features

I've looked at a couple linux phones and those tend to be designed with hardware switches for antennas and cameras, which I would argue are more secure