I totally disagree with this. It’s been made clear that Dean was John’s good little soldier growing up. IMO Dean wouldn’t have interferred if John were to throw a punch at Sam for too much sass growing up, or as punishment for running away.
Now if John had ever totally lost it and it looked like John would have beaten Sam to death Dean would have interferred, but throwing a punch or two during an argument wouldn’t have been protested IMO. It would have been seen as an acceptable form of corporal punishment within the context of their twisted upbringings.
Plus just look