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 wonder if some of it is fluffing the metrics too, like “Since we announced that third party apps are going away, we’ve had X thousand downloads of the official Reddit app” (meanwhile not mentioning that they’re forcing a majority of mobile users away from the mobile website)