The 2nd point is that, most pros don't rely just on aim to win duels, since many players have good aim. Most duels are won with better positioning and awareness. Hence, M4A4 is more versatile, especially because of this trick:
And he described a trick used often by Astralis players with M4A4 - a special type of crosshair placement where you aren't even looking for the 1st shot headshot. You aim at the stomach/low chest area as you crouch peek, and then walk. That way, if opponent was aiming at your head-level, they will have to adjust aim and move crosshair down on your head, but stomach is a 2-shot kill while chest is a 3-shot kill (so higher firerate of A4 helps), so you are at an advantage. Most importantly, if you aim like this, if you leave crouch button, your crosshair will automatically land on a standing enemy's head (physics), whereas if he was also crouching, your crosshair will already have been at his head (since you were aiming at stomach height).
He just explained this and lots of other nerdy stuff with live demos and all. I just remember this, cuz it sounded like a really neat trick xD.