IIRC the official app is from them buying AlienBlue, which used to be the best one on iOS. These days it's Apollo.
Apollo recently moved some push notification features behind a subscription to cover ongoing costs, separate from their "Pro" one time in-app purchase, so I think Reddit is at least taking a cut from API usage by other clients.
Weirdly, most of the features Apollo puts behind the subscription tier are features I think foster unhealthy engagement with Reddit and I'm just as happy to not have hanging over me.
I'd be happy to pay them, but I actually don't WANT push notifications, posting from the app, or any of the other stuff that encourages websites to pull you in deeper.
And the only reason I even use Apollo is because mobile Reddit is a nightmare UX. Literally a case study in how to make a horrid, horrible experience on the mobile web.
I don't use the iOS versions, but IIRC they never really did anything with AlienBlue after the purchase, and the "official" mobile app is a totally separate project.
Apollo recently moved some push notification features behind a subscription to cover ongoing costs, separate from their "Pro" one time in-app purchase, so I think Reddit is at least taking a cut from API usage by other clients.