Everything's federated after any user tries looking at something there, and only gets defederated after the admin adds it to a block list. If the admin's not aware of the noncery, they won't have defederated it yet. If it's not defederated relatively quickly, I imagine most people here will switch to an instance that gets rid of things like that more quickly.

As for why it's not been noticed but the tankies have, it'll be because it's really easy to accidentally run into lemmygrad.ml posts, as they're getting upvoted and commented on faster.