[Jochim]

The implication is they buy the app and do what they like with it.

The alternative is that he has no choice but to shut down the app, given that they've announced what the price will be 30 days before it's introduction. Even if he'd said nothing there would have been a shitstorm; the timing would be obvious.

Reasonable notice of the price increase would have given 3rd party developers time to monetise and meet the new costs. A more reasonable price could have been borne by 3rd party apps with very little fuss. Making API access a premium Reddit feature would have put even more money in Reddit's pocket. Buying out the 3rd party apps would have been unpopular but would give Reddit the appearance of being less incompetent, underhanded, and duplicitous.

Instead, Reddit made literally the worst possible choice in this situation: alienating their users and the moderators that do most of the work on the platform.