because he repeatedly states that profit is not a motivator, but then that menu screenshot of the app shows a "Remove Ads" button

Profit may not have been a motivator, and may still not be. But if he can make some on the side with very little effort and without detracting from the other motivators (presumably the challenge/fun of creation and improvement which is what I think most of us in software will cite as our main reason for keeping at it) or offending his other values (some developers won't have adverts near their output if they can help it, even if it is the only way to make something out of it and making money is a an active desire) then why not (either through ad-sponsored methods or directly paid)?