This page seems to describe installing Plasma Mobile on top of Cyanogenmod, a fork of Android. Neon is mentioned only under "OLD INSTRUCTIONS AND ERRATA (no longer needed)".
Minimal Cyanongenmod base refers to the kernel and some blob drivers, there is nothing else there from CM. So it's actually normal glibc Linux, which uses some hardware drivers for Android through libhybris.
> * https://plasma-mobile.org/technology/
This page doesn't mention the OS, afaict.
> * https://plasma-mobile.org/nexus-5/
This page seems to describe installing Plasma Mobile on top of Cyanogenmod, a fork of Android. Neon is mentioned only under "OLD INSTRUCTIONS AND ERRATA (no longer needed)".