People don’t claim that the OS can’t access keystrokes or text they display. Those are nearly impossible to prevent, and if they were prevented in the DRM style, by having them handled by a separate OS… then guess what - you still rely on an OS. You can’t escape having an OS, and no one is claiming that you can.
The OS may not be as private as unfamiliar people believe, if that belief is impossible.
Since you can’t escape an OS, you should be handing your data to one that you can trust or audit, and that’s where Librems win.