See, when I read that, it dovetails into exactly what I see if the problem when people are discussing "player agency."
What, exactly, do you mean by player agency? To you, and in that example, the concept of player agency in that example is nothing more than "Rules are followed,
despite what the fiction allows." On the other hand, a person might reasonably say that 4e constrains player agency to the extent that ... wait for it .... "
Rules are followed, despite what the fiction allows."
So while you might credibly say, "Look, a player has this cool ability. He should be able to use it, even when it doesn't make sense. The GM must accommodate the player in order to
increase player agency," another person might look askance at that and say, "Sure, but in that ruleset, the player's agency is constrained because while they might be able to 'trip' a gelatinous cube, 'tripping' is a defined function of the rules, and it can only be employed in the exact manner of the rules, no matter what the player wants and the fiction allows. Therefore, those rules deny player agency."
In essence, it's nothing more than rehashing an old debate about the utility of certain specified rules while employing the term "player agency," which doesn't add to the conversation. It's a terrible term because it's essentially a meaningless term when people use it.
The most notable feature is when you say that "
A more narrativist or agency-supporting approach ..." Here, we see what was earlier discussed with
@TwoSix in the analogy with sports (and defining "player agency" in terms of "ability to influence winning"). What you are doing is simply using the term as a synonym for narratavist, which means that the term add no value- just like saying that an amateur bowler has more "player agency" than LeBron James because that amateur bowler is playing individually.
As far as I can tell, having seen this argument countlessly replayed, it's a completely pointless term that doesn't add, and usually subtracts, from any conversation.