Or just clone whatever API the official Reddit mobile client is using. As long as it's offered for free to the official app, there's no technical way to stop another app from using the same API. The best they could do is bundle some private keys in the official app, but ultimately anything on the client can be reverse engineered and cloned by another app.
The only solution Reddit has to that is complaining to Apple, who can reject the third party app from the App Store. There's precedent for this with things like "unofficial" Pokémon Go clients. Apple is usually happy to remove them. But I'm not sure it's ever gone to trial - it would certainly be interesting, given case law around APIs like with Oracle v Google, or LinkedIn v HiQ.