Long Boring Video on Tank Destroyer History

ArcherII, on Aug 30 2016 - 17:14, said: I caught this on youtube and enjoyed it a great deal.  Like the U.S. Army flag in the background.
  Two questions: - Would it be fair to say that the TD concept as originally envisioned was almost purely defensive in nature? - What in the hell is going on in the background because it sounds like a party you just weren't invited to?