Sounds like they are becoming worried over the growth of these networks and wants to convince the large public that they should stay away.
It's pretty much standard tactics to paint a false picture of something, and they get away with it too. I bet people will now say "mastadon, isn't that where there is child porn? No thanks".
I was thinking that too. It would be a lot easier for new users if they were already subscribed to some communities so they get a feed with posts and can get started, ask questions, add or remove communities etc.
Maybe not tie it to Lemmy explorer but at least let people have some starting point.