I’m not sure why his ability to impersonate Grogar is so unbelievable. Discord is one of the few there old enough to possibly have known the real one, and even if he hadn’t, none of the other villains recruited ever knew Grogar personally. The only thing Sombra mentions is having heard of him. Tirek and Chrysalis remember tales and legends about him, with some doubts as to whether he was even real, and Cozy Glow doesn’t know who he is at all and doesn’t seem to have heard of him either. It’s really the perfect part to play.
Nevertheless, Discord had revealed important information about Grogar, who is called “the father of the monsters” and was a notorious villain who had ruled over what became Equestria in recent times. There is a strong possibility that Discord could have imitated Grogar because he was created by him, and thus put with intimate knowledge of him and his magical signature that we know is real by observing the bell.
“Grogar Forgery” as I call it, was the biggest disappointment of the entire Season 9 and one of the most controversial events of the entire G4 that had led to strong feelings among fans and viewers. It was a great shock, more experienced writers would have steered away, yet they did not do so with Grogar and Discord. We were expecting a grand battle between the good and the bad, where Twilight and her friends had to fight what was considered the biggest villain in G1 at that time.
Instead, Discord messed up, disappointed many who liked his character development and personality, his arrogance, indifference as when Sombra terrorized Equestria and destroyed the Harmony tree, and short-sightedness threw him into the abyss of disaster. The Mane 6 and her allies and friends had to clean up. And when the three villains are punished, Discord once again had to disappoint his fans because he went back on his word about not petrifying ponies as seen with Cozy Glow.
The final episodes of just 22 minutes are too short to see if Discord is being punished, he had committed criminal acts after all and allowed the three villains to be punished, which could be a criminal offense under the law of most human states. Because it was Discord who released them, encouraged them to commit offenses, and gave them the bell. It is punishable and the basis for acquittal in our world. Why did younger viewers see this?
Even in the Middle Ages, human beings were not tolerant of the kind of organizing the crimes of others. Here the writers had erred, they completely forgot that we have laws that would have made Discord an accomplice in the crime and that his actions are morally unacceptable. Although Discord risked his life as he helped out the rest of the Mane 6 and was forgiven by Twilight who chose to release him - many would not be disappointed if he had been left in the dungeon - it is a moral requirement that he be punished .
For the sake of viewers.