He has 50,000 customers who paid $10/year for the app. Now he's put into a position to support those customers at $2.50/month. (He estimates their server cost is $0.10 per month.) That's an instant $125,000 per month out of his own pocket that he can't recoup from existing customers for at least the next 6 months.
Over the course of 2023, he'll have to pay Reddit $1 million MORE than he has made from the app this year.
Reddit doesn't want to work with third-party apps. That's fine. That's their right. But it's certainly not the app developer's fault that he's forced to quit.