He shouldn't be insulting you or anyone else. That said, things do change. Not just in storytelling, but in real life. There was a time we thought we were at the center of the universe. There was also a time when people thought it impossible to go to space. Just because something was once true, doesn't mean it will always be that way. But in regards to fiction, you can point to so many examples of the lore of a setting changing, for better or worse. People might call it a lore retcon, I would refer to it as the lore evolving. Obviously if it's badly written, lazy, and makes no sense, then yes I would refer to it more as a retcon. But if there's a lot of though put into it, and it serves a purpose in the story, then it doesn't necessarily mean it's a retcon.
The Witcher 3 establishes the process of the Trial of Grasses is nearly forgotten, and Witchers are on their way out. However towards the end of The Witcher 3, another Conjuction of Spheres occurs, potentially bringing in more monsters into the world. It stands to reason that there will be a need for a new generation of Witchers. How will the Trial of Grasses be rediscovered? We don't know yet. How will Ciri survive it? It it because she's unique? Is it because it will be a new variation of the trial that can allow both sexes to survive? We don't know yet. But seeing as how CDPR have mentioned that they will explore this, I don't think this is really worth all the time and energy to get mad about it, until we see what they do with it.