We can build this out a bit - agency requires action taken to produce a particular effect. If I didn't know the ogre existed, a choice to take one path over the other isn't agency, because that action was not taken to produce the effect of missing the ogre. Choices made in ignorance do not provide agency.
I don't think this claim is true in general.
Suppose that, for some reason,
going left rather than
going right is a thing that matters to the player. For instance, perhaps their PC has a blessing that is triggered by going left. Perhaps the player has authored a Belief for their PC such as
When I have a choice, I hue to the left path.
Now neither of the above is very likely in the context of D&D play, I will admit. (Though I don't think either is impossible in a D&D context.)
But suppose that the game being played is dungeon-crawling D&D, and the players are carefully mapping the dungeon, and from their map their is good reason to think that by going left rather than going right, they are more likely to fill in useful details of their map that might reveal the parameters of some suspected hidden architectural feature (eg a secret room that they have good reason to suspect is there, given the shape of other elements that they have mapped).
Now we have an example where choosing one path over another may be quite agential, in the context of game play, although the players do not know what their PCs should expect to find (and maybe the GM doesn't know either eg if they are using a die roll procedure to generate wandering monsters).
Once we recognize that agency depends upon what the player wants, it becomes clear that it is a subjective matter.
Why would I recognise that agency depends on what a person wants.
Agency is about
capacity to bring about change.
Capacity to have an impact. Manifesting agency requires having a goal, insofar as the manifestation of agency occurs by dint of making a choice and acting on it. But the presence or absence of agency is not a subjective matter.
One of the most famous literary treatments of this is Brave New World. Part of the point of Brave New World is that it is a world in which nearly everyone gets what they want, but almost no one exercises agency.
Adaptive preferences, as discussed by Sen, Nussbaum and others, are another example that illustrates the contrast between
getting what you want and
exercising agency.
In addition, even if we ignore that contrast, the fact that
what a person wants is subjective (as in, personal to them) doesn't entail that
whether or not they are getting what they want is a subjective matter. That's an objective matter of fact.
there's no guarantee of being able to produce that for other people at the table with you - in rather the same way that your personal favorite pizza may not be good for everyone else at the table.
I don't know what you intend by "guarantee" here. If all you're saying is that some players may prefer a more agential play experience while others may prefer a less agential one, that seems like a fairly basic truth that tells us nothing about what constitutes high or low player agency in RPG play.
I expect this is part of why Snarf keeps noting that talking about agency broadly, in general is not useful. Speaking about specific techniques, forms, and modes that can produce or enhance agency becomes more constructive, because producing agency becomes an act of assembling compromise solutions.
I don't see why we would being our discussion of RPGing techniques from the premise that the players have inconsistent premises. We don't generally do that in analysis of other games.
And frankly, talking about player agency in general is quite useful for me, because it permits me to describe various approaches to RPGing and to say what is or is not appealing to me about them.