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.
Finding and subscribing to communities on other instances is not slick enough. I hope it improves. It would be nice if people could discover and subscribe to communities without even having to think about which instance they are on.
Other than that, everything seems very straightforward.