An immutable OS is fixed and mounted non-writable. Every update you get, every program you install is handled on top of it via containers or filesystem overlays so the underlying OS is untouched. Basically the same concept you know from smartphones or other devices with a “reset to factory settings” function. No matter how hard you screw up your system, you can always reset to the base OS, either by granulary deactivating things installed on top, or by a reset to the working base OS.
And just lke with the war on drugs, countries will realize it’s a lost cause. And will then instead try to coopt the system to spread their own desinformation. If you can’t win, exploit it…
Welcome to our wonderful post-factual age.