As long as Valve is committed to an open system, without locked down bootloader like on mobile phones, it is unlikely that kernel-level anti-cheat can be implemented.
But that also means Steam Machines are unlikely to support 4K streaming from Nextflix and co. because also DRM will also only be on the level of other Linux systems.
SELinux protects systems from bugs in software. Not against users with full root privileges using their own hardware.