It's very clear the MoCo doesn't reinvest all the money they make (they have a lot in the bank). And some are getting rich from it, but that's a different topic.
And yes, the MoCo is not reinvesting all money all the time right away, but they'll have to reinvest it or use it to pay wages at some point. They have no reason to just amass money senselessly.
If they make a surplus, have all wages paid, and don't save it up nor reinvest it, then they can only pay it out to the Mozilla Foundation, where it's again in nonprofit hands.