Gregory's Server
When the "news" trending feature (without the author credit) first launched, instances decided for themselves which sources they wanted appearing based on their instance policies. The actual sources didn't need to email anyone, the admins simply decided based upon the reputations of the sources.
As far as I can tell from the github it's the same situation with the author credit, but the Mastodon blog post has done a really bad job of explaining it by adding the part about emailing them. The blog ought to be about Mastodon software, not mastodon.social, but the blog is treating the two things as one thing.