The answer lies in both the first part and the last part of the post-credits scene. Remember that video the unnamed French doctor was watching? Well, it was a transfer from none other than… CDC virologist Dr. Edwin Jenner (Noah Emmerich)! Long time Walking Dead fans will feel Dr. Jenner remembering his performance all the way back in The living Dead season 1 finale. At the time, he said the French were making convincing progress in finding a cure for the virus.
“It was the French. […] As far as I know they were the last to last. While our people left the doors and committed suicide in the hallways, they stayed in the labs until the very end,” Jenner told Rick Grimes and friends.
In this broadcast from Jenner, we see why he was so excited about the progress of the French. Jenner says he’s looked at the French scientists’ data and that he “likes their approach”. What follows is a lot of heavy jargon with terms like “activating systems to counteract CPR,” “priming circulation,” and “short circuiting the brain” to deal with “variant cohorts.”
To the layman, it seems that what Jenner describes are the attempts of French scientists to shock the systems of infected individuals to stop the zombification process. This seems like a good idea in theory, but like any Dr. Frankenstein can tell you, introducing energy to resuscitate a corpse can make it a little stronger than you expected. Perhaps that’s what Jenner is referring to when he talks about “variant cohorts.”
As we are all very tragically now known in the real world, any virus worth its salt will learn to mutate into new variants to outshine the immune systems of living things. Appear to have been resuscitated with the “zombie virus” on The living Dead almost always behave the same way, but we’ve seen a small handful of anomalies over the franchise’s long run. Think back to that time in Season 1, some zombies (like Morgan Jones’ wife) showed some sort of primitive reminder of their time pre-zombification. Now on Fear the walking dead, main character Alicia turns out to be infected with a zombie bug that progresses in an unusually slow spot.
Then, at the end of this post-credits scene, we see another example of the unusually working zombie virus. The gunman shoots the Doctor in the back and within seconds of her death, her zombified corpse springs into action, rushing to the door and clawing at it vigorously as if trying to avenge its death immediately. Quite simply, we have never seen a zombie operate with this level of speed and ferocity.