As quoted from the linked post.
It looks like you’re part of one of our experiments. The logged-in mobile web experience is currently unavailable for a portion of users. To access the site you can log on via desktop, the mobile apps, or wait for the experiment to conclude.
This is separate from the API issue. This will actually BLOCK you from even viewing reddit on your phone without using the official app.
Archive.org link in case the post is removed.
The site worked well enough on mobile… Reddit would say it worked too well. Sites can’t collect nearly as much data about users as apps can, so they’ve got an incentive to try and get people to use the official app. The app isn’t good though, so the only way they can get people to switch over is by making the site worse, and forcing people to use the app.