Yeah, I work at a big international tech company myself and sometimes get surprised by the amount of people we have working on our app.
My guess would be that it has to do with
More features (I think the Apollo developer mentioned how some features aren't available through the API?)
More insights (I would guess that the official Reddit app contains way more code to track and quantity the user)
On-call. I assume the 3PA won't get paged in the middle of the night if there's a critical bug in their code.
Experiments. Wouldn't surprise me if the official Reddit app is using experiments (I.e. A/B testing) to try new features or changes in UI
New features. The Apollo developer is the one who has to adapt. API changes, new features are (maybe?) added while I assume in house app developers work together with the rest of the company to bring those features.
Etc.
None of these points explain why they'd need 80x more developers of course. It's also just because reddit is a big company and the bigger you get, the more time you spend in meetings, writing documents, etc. and then you hire more developers to increase the velocity and then you end up with a slow machine.