Discord was originally a place to organize for playing videogames with people. If you're using it for that purpose, you're probably on your computer already (because that's where the games are). Thus, the mobile app ends up being more for checking in with your community while away from the computer, rather than a primary driver of your engagement with the platform.

They could have been using it in browser. One of discords underrated decisions is that the browser version is fully featured and doesn't force you to download the app, a new user can join a server in their browser with just a name which is as low friction as you can get.