You do not recall correctly. I hunted down the right point in the right video, here's a
timestamp link.
Crawford starts by talking about the subclasses as thematic quartets, segues into talking about how
sometimes those quartets are broken into pairings, and then veers back into quartets and how the theming is sometimes aesthetic and somethings mechanical. Then he has this to say. "An example of a class where the hooks tend to be more mechanical than aesthetic is the Fighter. Where the quartet in the Fighter is actually about four very different ways of play."
People here and on reddit had already been speculating about the subclasses being broken into thematic pairs, so that's the bit that stuck in everyone's head, but it's not a hard or universal design rule.