That's a crazy ass dunk, but also a good example of how a big fish story can get started. Had that video not been there, those guys would probably tell their friends "oh my god Dwight Howard made this one dunk that was soooo sick. his freaking head was over the rim and his hands were at like the top of the backboard. seriously, i'm not kidding!". Word would spread and suddenly it would be fact that Dwight Howard could touch the top of the backboard.
In reality his head never went over the rim and his hands were just a smidgen higher than the top of the square. The top of the square is (only) 11'4" so let's say he got 11'7" to be generous. Still got an extra 1 foot 5" to the top of the glass. Now since we've seen the picture of him "kissing the rim" let's assume he can get a little bit higher than this video and say that in the kissing picture he could have touched 12 feet if he had tried. Still, he would need a whole extra foot to touch the top of the board.
When you break it down it really seems harder than it should be.
First of all, the angles in that video distort the perception of how high he is. Second, that video doesn't show the ball at the top of the backboard anyway. The last angle is the most accurate, and shows his hand about even with the top of the white square.
Since there's no documented video of anyone doing it, I think all of those playground legends (Jumpin' Jackie Jackson, Earl Manigault, and later people like David Thompson) touched the top of PLAYGROUND backboards--those round, lower ones...
They could with trainning, very few athletic big men.
But they would need momentum meaning timing to block would be tricky as hell, not to mention the chances of injury on the way back, specially back injuries on landing, or just landing on someone's body part and losing balance. Or someone touching you on the way up, that would be crazy.