Np, happy to help. I had the exact same question a few days ago.

At the end of the page, theres a link named Instances, there you can see which instances are linked or blocked with the one you are using.

It's blocked in your instance sh.it.just.works. Not in lemmy.ml. Each instance server has their own list of linked servers and blocks.

The one blocked in lemmy.ml is a troll instance name, it doesnt redirect to lemmygrad but to a pro trump one.

Don't worry, the dev of redreader said that he has been in touch with lemmy devs and plans to make the app more general, such as able to interact with diferent sites: Link to his last update on the app