Monetization and growth go hand-in-hand. I'm an old.reddit.com user (even on MoWeb, I know I'm a psycho), but the way people talk about Apollo is like it's absolutely superior to the current Reddit app. If I were Reddit, and my users loved this third-party product _so much_, I would at least explore promoting Apollo to a first-class interface for browsing Reddit.
For the scale of Reddit as a company, it's likely a trivial deal; whereas the cross-pollination of ideas and UI/UX learnings could easily be worth more than the cost of collaborating.