I have been a Fediverse (the Fediverse predates Mastodon) member since 2009, and I have almost never seen anything that would be illegal in the USA. Each nation has different rules, so for example, I don’t follow anyone from the big Japanese instances, because they allow things that other nations don’t. Likewise, the USA allows things that many other nations don’t.

This article is mostly FUD. If you don’t follow or interact with people on instances that allow such things, you never have to worry about running into it. I also think almost every instance has a “we call the cops” policy for such situations.