This seems to be a theory based on the assumption that the person(s) are already having more than their base needs met: (decent housing, food, entertainment, retirement). Salary is one hell of a motivator if you're struggling to meet these things. It's all good and academic but antithetical to why a good portion of people obtain higher education.

It is just an academic theory, not holy gospel (with a long "Validity and criticisms" section in Wikipedia).

I think salary IS a motivational factor in real life, as for many humans it is a yardstick of superiority compared to other humans (whether it is right or wrong is another matter).

Have you ever asked someone if they would still be working at their job if they weren't getting paid? Or even the weaker version, if they didn't need the money?

I have asked many people this and only two people have said yes to this. One was someone who already sold his startup so probably didn't have to work ever again. The other was someone who could have retired but he was afraid if he shut down his company his employees would struggle to find new jobs.