If idle on Win 11 already pre-caches most-used apps like Chrome/FF and not on Linux, how can you accurately measure what true idle is?

You have to remember the Win (and MacOS) kernel is tuned to consumer desktop user use cases, while Linux kernel is tuned to server/DC use-cases.

That's why ~15 years ago when I was in uni, you'd get audio stutter when playing music/movies of spinning rust on linux while torrenting file on the same HDD, and not on Windows/MacOS, due to different queuing algorithms used thet prioritised audio decoding tasks over everything else, which Linux kernel out of the box didn't, unless you tinkered and changed it.