"Extremist" is just a pejorative variant of "radical". I assume they're using it tongue-in-cheek.
When it comes to speech, it's really not hard to imagine positions that would have been controversial at any point in the history of the US. That doesn't mean you can't hold them, but others don't need to agree, and that's how you end up with labels of this sort.
Aside from the bit about "frontiers", Article 19 of the UN Universal Declaration of Human Rights is pretty straightforward:
> Everyone has the right to freedom of opinion and expression; this right includes freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers.
As is the First Amendment to the US Constitution:
> Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
I can't speak for Pete. However, given that the expressed position of influential portions of the US government (as well as many of my peers and acquaintances) runs counter to the letter and the spirit of Article 19 and the spirit (if not the letter) of the First Amendment, I consider myself to be a free speech extremist.
Because there are inumerable forms of banned speech. Because freedom of speech is in reality a very narrow construct. See Hustler Magazine v. Falwell 1998.... or just watch the last few scenes of the movie.
When it comes to speech, it's really not hard to imagine positions that would have been controversial at any point in the history of the US. That doesn't mean you can't hold them, but others don't need to agree, and that's how you end up with labels of this sort.