It's not just that. If I'm accessing all of my reddit content through the app developers api pull requests they can't track what I'm doing except for when I comment/post/vote. They can't tell how long I spend on the site, where I'm scrolling, what I'm looking at, nothing. All of those API pulls are through the developers account, not mine.
So not only am I not looking at their ads, I'm also not giving them any information on what I'm doing at all, so they can't really give that information to their advertising partners.
Then of course they don't have the ability to send me random alerts on my phone to pull me back into their app.