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.
Honestly, mobile browing (even using old.reddit) has been garbage for years because they detect your mobile OS and constantly try to push their app on you. Click on link, do you want to open in mobile app? let me open the playstore for you. And then you also get limited comments. To see more comments open in mobile app… You could do a case study in how to alienate your customers into leaving your platform on just mobile browsing reddit.