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.
I didn’t get the notice, but scrolling through a sub yesterday, I got a notification that this action (viewing a subreddit) is only available for logged in users on the app. Seriously?? Just to view a sub? I reloaded the page as desktop site, and it allowed me to view the sub, although formatted terribly. Reddit is getting much worse than I thought possible.