He said on a podcast that he spent "6 months" developing it. I very much doubt that was 6 months of full-time work, but even if that was only 10 hours a week, and even if the "cost" was let's say $100 (probably much more) an hour, that's around $25k or 7 straight years of $300 a month (that never goes down for 7 years, aka impossible) to recoup his investment. That's an absolute low end, I think you could easily argue the cost was 4x that, which means it would take 28 years of sales that never dip to break even. There's no possible way he could make anything besides less-than-nothing level money at $300 a month or anything close to that amount.

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.