I think the problem is most peoples’ definition of retcon. It used to mean a clear change of a clear fact, now people use it any time a story goes “Ah ha! It’s not how you thought it was, surprise!”
With the wrathgate This was one dev’s perspective. It’s how he’s interpreted the story. There is no proof of it, but it’s how he saw things. Keep in mind he has a more overall perspective, his view could include story that was cut or never released, and that made more impact on him. To the majority player base, we are led to believe it was Putress who was mainly responsible.
But that’s another point, when a story is vague, when we don’t have 100% proof of something, that leaves room for hidden story to be expanded upon further. It’s not a retcon, it’s revealing the rest of the picture.
As for me personally, the one hidden revelation that’s bothered me most, is the Orcish Clans of Warlords of Draenor. Any existing lore about the old clans from the true timeline’s draenor, like in vanilla, WCI & 2, books, is just forgotten. Ever since Warlords of Draenor, the Alternate Timeline, everyone considered these clans and their histories to be the end all be all of orc history.
But I still don’t consider it a retcon. Neither is Illidan’s secret motives we don’t learn of till Legion, neither is Sargeras’s motives for destroying the universe. We originally thought he was driven mad by all the evil he’d been forced to defeat and imprison in the universe, causing him to rebel and destroy. Azeroth’s heavy use of arcane magic is what we thought brought him here. But again it’s all a matter of perspective, as we learn, we expand our perspective.
Edit: One more thing about his Wrathgate comment. I also think this was an off handed comment, basically a mistake. This is how he remembered it, how he felt about it, from some writing that was never released or hasn’t been released yet. I believe if he studied this question before an interview, he’d know exactly what information was given to the players, and what we were all told to think, that Putress was responsible.
That being said, his comment isn’t technically wrong, and wouldn’t require calling it a mistake. Because Sylvanas and Putress could be responsible. She did order the creation of the plague, that was a deadly weapon. Even if she never explicitly commanded Putress (I believe he was in fact separating from her), she’s still responsible because she gave him the means to his terrorism. She created a “WMD” but didn’t secure it adequately. It’s why countries fear Nukes so much, they aren’t necessarily afraid of another country firing them, but if they ever got in the wrong hands…