To understand the exclusive, you have to go back to the beginning. Before the TV series, there was the 2007 pilot. It was crude, loud, and animated entirely by Pendleton Ward himself. For years, this pilot was the "Holy Grail" for fans. It wasn't officially available on DVD or streaming. The only place the general public could reliably view it was the Internet Archive (archive.org).
Various versions exist on the Internet Archive, including the raw Nicktoons pilot adventure time season 1 internet archive exclusive