This is because a stun/stagger always beats a knockback. Doomfist does not actually use a stun but instead uses knockback(punch), knockup(uppercut) and stagger(slam). Reinhart also uses a knockback and they both cancel each other out when they hit. (Knockback vs. Knockback)
Brigitte uses a stun and that means it interrupts movement/actions.
Tip: if Reinhardt is charging you instead of punching him use your slam than uppercut him and finish him off with a punch.
Summary: Doomfist punch is a knockback and is lower in the cc pecking order than Brigitte’s sheild bash which is a stun. I’m sure you were just confused. (as so many other people try to use that argument)
The doomfist punch itself does not stun. The Collision with a wall is what stuns somebody. So until they hit the wall it is just a knockback.
Sorry but you have not proved anything other than you think a knockback is the same as a stun.