A very comprehensive SELinux deployment for one.

SELinux will stop any process in android from loading kernel modules, that’s not allowed. The android permission model as a whole is ultimately backed by SELinux.

Not sure what specifically they're referring to, but Android (and iOS) add a lot of sandboxing to ensure that each application can only access its own files, can't access hardware willy-nilly (bluetooth, scanning wifi, etc), can only link against certain libraries, etc.

Imagine if Linux only let you run stuff from Flatpak, and if stuff didn't work in Flatpak then too bad for you. Most Linux users would hate it and it would be a mess a lot of the time, so, for user experience (UX) reasons, they don't do it. Android can get away with it because that's been the app paradigm for decades now.