Was he not on board with 4e's design principles? AFAIK, the seeds of 4e's design were sown with the Book of Nine Swords, where he was a writer. He was also part of the Scramjet, the core 4e design team that looked into all of D&D's sacred cows and decided which ones should be offed. Surely he would've taken the game in a different direction if the main tenets of 4e didn't appeal to him at the time?
You are correct, and this is just revisionist history.
Mearls was added to the 3e design team, and pushed 3e in a certain direction that led to 4e. In fact, just before joining the 3e design team, he wrote an essay (which I have quoted before) which introduced the terrible, no-good term "Mother May I" into TTRPG discourse.
He was, from everything I understood, one of the leading proponents of introducing more narrativist and "Forge"-like concepts into 4e. This is hardly surprising, given that he was one of the people that was heavily involved in the discussions that led to it.
In short, while I am sure we will get more information from Riggs in the future, Mearls was arguably responsible for a lot of what people
liked about 4e (in terms of the different direction that it took).
So ... why the backlash? Because Mearls was one of the few people that stayed on after the bloodletting. At that point, despite what people want to say, 4e was considered a failure. I don't mean that in terms of "a well-designed game." Or in terms of "providing an awesome experience that people still like to this day." I mean that in terms of "a broadly successful game that would ensure D&D's continued dominance."
Which means that when he was speaking post-2010 (when 4e was internally dead), he was wrestling with why 4e failed. For people that were still engaged in the front-line skirmishes over the "edition wars" of the time, this probably seemed like a massive betrayal. Remember- they had been out there, defending the game, and here was one of the designers talking about the issues. But for Mearls, it wasn't so much a repudiation of what he had previously believed, but an attempt to reconcile the fact that what he wanted was not, in fact, what was broadly popular.
At least, that's the way I view it. Another way to look at it is that, quite simply, his paycheck depended on him figuring our what went wrong and fixing it. And internally at Hasbro, regardless of how well-designed 4e was, it was something that had gone wrong.
I think people often confuse something that is well-designed, or even ahead of its time, with something that a company should do. There were a lot of people that loved the Apple Newton. But it wasn't a product that Apple should have released at the time, even though ideas from the Newton ended up being adopted into a lot of later products.