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.
Testing changes on a few users is a common thing and a good practice to see the general acceptance of changes. Its what you should be doing.
Rolling out a new feature to a subset of users is fine. This is removing basic functionality from those users
What changes exactly were they testing?
How easy it is for users to circumvent their blocking most likely.
I wonder if the AB values are stored in a cookie…