Cody's promo didn't break any kayfabe because it didn't expose or hinge on the predetermined nature of the business.
Within the bounds of kayabe, heel is an insider term used for wrestlers that act like bad guys to get attention, rile up the crowd, or psyche out an opponent.
Dwayne referred to himself as a heel as a way saying "I'm not being sincerely mean to Cody and his mother. It's just an act. Please don't boo me." It was an attempt to make himself still look good as a famous celebrity/Hollywood star.
Cody, in his promo, called BS on this. He claimed the Rock is not "a heel" in the sense that it is an act, but rather that Dwayne truly is a despicable piece of shit.

This isn't that hard to understand.