I disagree completely with that, at least for software engineers.

The work queue is infinite, measuring output with any sort of precision is almost impossible for a lot of the work (maintenance, actually necessary refactors, security, mentoring juniors, managing stakeholders, etc etc etc). There's no "I've finished my work for today".

I'm not saying that I like it, or that it's a good thing, but in my understanding engineers are paid to be available doing work roughly 40h/week.

All of what you mentioned is quantifiable.