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.
They want to force more people to download the app so that they can show potential investors how many people are using the app, and so they can mine data off of phones with the app. I fucking guarantee that’s what this is.
They are gambling that the people who incidentally land on reddit using their phones to search for things will be more likely to download the app than stop using reddit when it comes up in a search.