What Cena said in the promo was for the most part the right message, but the delivery was pretty awful. If he’s going to be a heel, then don’t have him whining. It was as though he was on the verge of tears.
The psychology of what they did made complete sense. It was very clearly crafted to make him completely unlikeable and fully turn him heel, rather than give the fans the fake "cool heel" where they are in on the joke so to speak.
WWE's audience generally plays along with face/heel dynamics more than AEW, but at the end of the day if he went heel how everybody wants him to, he would basically be turning face and while the match would still have interest, Cena playing along and being the cool heel everybody wanted him to be for 20 years doesn't help maximize the interest in Cody VS Cena.
They need him to get the upper hand physically on Cody going into WrestleMania, and they have to present him as a viable threat to the title reign, but in terms of character work to make him completely hated, what they did was perfect psychology.
It was a bit cliche "you people", etc. But he had a very clear job, which was to go out there and have everybody wanting to see Cody kick his ass. Up until this promo, the fans were in on the joke so to speak. The heel turn was a big news story, but it was a cute thing they were playing along with. Now he actually has heat after that promo that they can use to maximize interest in the Cody match.