Linux has gone mainstream a long time ago - from dvd players to android phones. It just hasn’t succceeded on old preexisting markets ( personal computers ) but has taken large parts of most of the newly created ones ( servers , mobile devices , embedded devices etc )
I feel like this is a little nitpicky. Clearly they are talking about mainstream for the average home computer user as a daily driver OS. Even with the steam deck it’s barely touching 5%. It’s still obscure and mysterious to the vast majority of the population in a way windows and macOS aren’t.
Linux kernel is an implementation detail on Android, not exposed to userspace.
Consumers don't care what kernel runs on their electronics.
Yes - but this will be the year of Linux on the desktop.