You have identified a major problem.
I never paid attention to the time management gurus, from the 1970s to now. I agree that attention management is now a problem for most people.
I have used a multi-tasking technique for years that is radically different from the normal task-switching ideas presented here.
- I give my brain a problem to solve.
- Go do something else, another task or read sci-fi.
- Go to bed.
- Wake up the next morning with the solution.
There are many ways to solve the same problem.