Who said anything about being superhuman? The actual quote was "Bruce looked at it and just went Bang, it shot up out into the lawn about 15ft in the air, it then busted in the middle"
While this isn't a common occurance, Bruce's explosive power was confirmed on several occasions from several sources. All different individuals from all walks of life. He was obsessed with exercise and working out, and for punching and kicking he routinely worked on a 300 lb bag to help develop his power as well. Now as to the quote you decided to focalize on, I think that there may be some embellishment, like the bag may not have been 150lbs and the height may not have been 15 ft precisely; but the idea that he could hit it and launch it isn't so far fetched because I've know 2 guys in my life that could do about the same.
I have broken a few bags myself with side kicks in my lifetime--never with a punch though. So what's so superhuman about it?
You seem to not just be someone that "believes in using proven styles" but actually someone that is "ANTI wing chun/Bruce Lee/JKD/etc." Why get your panties in a bunch over it? Some people achieved great things in their lives like Bruce, and others have been able to make Wing Chun work for them in application. So what?
YouTube - Bruce Lee Punching Heavy Bag