As he explained on his podcast, this project was designed to solve his own problem. The problem with apps designed to solve your own problem is that very often, your problem is not 1) shared by others willing to pay or 2) you don't solve the problem in the same way others want to solve a problem.
And “time isn’t money”. It wasn’t like the time he spent developing it would have been spent working on something else.
And time is definitely money, or rather, money is time. If you already have an income stream that is more than enough for your chosen lifestyle, spending lots of time on something that might give you a tiny bit extra seems like a bad choice. Why not spend time with his family, travel, take up a hobby, or at least write an app that is just for fun. Or one that has some sort of chance of making it really really big. Obviously up to him, but still, hard to understand.