People working in non profits typically earn less, yes. In general, salary is not a function of how praiseworthy or important or even hard the work is. People who work for non profits have pay cut, because basically they are willing to be paid less in exchange of doing something they see as meaningful.

(Excluding purely "money washing for local mafia and politicians" non profits.)

Dude, Lichess is entirely funded by donations. There's only so much money to go around.

And Thibault iirc is the kind of person that's not terribly interested in earning lots of money. Of he wanted to, I'm sure he could make bajillions elsewhere in tech, because he's that good. But he apparently prefers to only work for a "measly" upper middle class salary and doing something he's really passionate about. And I thank him for it, because lichess is awesome.