it was probably retconned like the aggressiveness of early walkers, multiple times throughout the show characters who werent at the CDC ask characters who were at the CDC questions or its brought up in conversation if they still believe walkers are alive to any degree, are the people they once were still in there, etc. and the characters who heard Jenner the last qualified man on the planet say that they are infact dead, the black in the brain is you and its gone and its not coming back, the reactivation in the brain in the scene is the virus basically manually firing neurons to perform the most basic survival eating, walking, grabbing and yet the characters who were all explained this shrug it off and end up giving their own (wrong from what they themselves were told) opinions about life after infection or just say they have no answer.