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 mobile browser that works better than their official app at that!
It also annoys me that if you click the “open in app” button it’s hard coded to take you to the official app and not just the installed reddit app (I have the redreader configured as my default which is handy should a website land me on Reddit unexpectedly)