I don't think we are going to resolve this, but the issue is the kind of agency you are talking about, can remove the kind of agency he is talking about. You lose meaningful choices when the players can alter things in the setting/narrative outside their character. Again I am not saying it is bad, or you aren't identifying another type of thing that is important, but it isn't simply a spectrum of ever expanding agency with ability to shape through character on one side and ability to shape beyond character on the more maximal agency side. A perfect example of this is when I ran my wuxia campaign murder mystery using Hillfolk. I get this isn't going to fit every example of what you are talking about because narrative systems all handle things very differently. But Hillfolk is a narrative system that I really like. But one thing that leapt out at me was you can lose meaningful choice in a mystery if players can shape the narrative (because it is hard to have a real mystery to solve when the players are able to invent the solution).
Again, I don't think we are going to solve this, it is a definitional argument and those never really get anywhere in these threads