"never designed to support third-party apps," yet it took them ~10 years to buy out a third-party app and call it their own "official app" (they bought out Alien Blue in 2014 and just rebranded it as their own).
Relevent section from Wikipedia:Mobile apps
In 2010, Reddit released its first mobile web interface for easier reading and navigating the website on touch screen devices. For several years, redditors relied on third-party apps to access Reddit on mobile devices. In October 2014, Reddit acquired one of them, Alien Blue, which became the official iOS Reddit app. Reddit removed Alien Blue and released its official application, Reddit: The Official App, on Google Play and the iOS App Store in April 2016.