Fair point, though I can imagine a lot of data they couldn't get from API usage (e.g., when an ad is visible to the user, how long does the ad spend on the screen, even if the user doesn't click on it?).
The confusing part for me is the extreme pricing--the ad revenue per user is magnitudes lower than the new API pricing (according to Christian of Apollo). If Reddit wanted to just make a profit, pricing out third party apps would make no sense (just charge double or triple the expected ad revenue, and make more than if they migrated to the official app).
Same with things like AI scraping. Scraping is easy enough for something like AI without API access, so charging a reasonable rate (and having companies actually pay it) would make more than the pricing they're going with (which will realistically result in non-API scraping methods).
Can't say I understand all the ins and outs, but I've been a programmer in corporate America long enough to guess at some of the things they can do by forcing everyone into their own app.