That's right, you posted the wrong rule. Then you tried to add words and interpretation into the correct rule. If a 3rd step is any act common to the game, why did the rule change add the words "a 3rd step?" There'd be no need to if a 3rd step was already wrapped up in the wording of the existing rule. This is what I mean when I say that people can't rail against a rule they don't completely understand or fill in things that aren't there.
Again, Dez was going to the ground. The thread starter here even says so and says it's idiotic that anyone could say he didn't (and he's right). When you're going to the ground, there's a rule for that and that rule takes precedence over part (c) of the catch rule you quoted above. So once you have that GTTG designation slapped on you, you CAN'T perform an act common to the game as listed in the upright rule ("pitch it, pass it," etc.) because you have to meet the standard of maintaining the ball through contacting the ground instead. He didn't, obviously, hence the overturn.