That's a helpful link, and segues to a comment I wanted to make - or a question, rather: how is agency measured? Measuring agency would be fundamental to knowing if I have more or less agency under some set of rules. As is my wont, I'm going to argue that different groups may both enjoy more of whatever agency is meaningful to them, while still potentially having less of whatever agency is meaningful to others.
With reference to the blog post quoted, I want to say that narrativism isn't about necessarily making more choices, because what counts as "more" depends on how you measure it, but rather more choices of a specific kind.
So the blog post isn't saying that sandbox play offers less agency than narrativism. It's saying that it offers the wrong kind of choices. That straightforwardly endorses my opening hypothesis.
The blog post overall says what the right kind are. There are two conditions. One it states outright - they're choices focused on human issues. This is about premises: problematic features of human existence to be resolved. The second condition is implied, but not overtly stated. I just want to pull in another quote from the blog to explain that.
It turns out the agency can be restricted. During play of the game, I cannot become a crooked cop. That's disallowed. Restrictions are basic to games: that's what rules are all about. Creating the distinct play, rather than just any play!
Does this mean that a sandbox where I could become a crooked cop offers more agency? Not at all. It just points out that the agency we're interested in differs. The second condition is in fact agency over resolution of premises. That's why
Freely doesn't mean "free to do anything", it means that players (not game designer, not GM) are free to resolve the premises however they desire. That is a powerful agency and distinct to narrativism. Coming back to my opening thought: I don't believe it necessarily amounts to more agency, because what measures "more" depends on what I am counting.
Is the measure of agency how many times I as a player get to say something that happens in our fiction or system? Is it the breadth of choices I get to choose between? Is it effect, or how powerfully I can impact the game world? As quoted from the companion blog
So no, it's not just any kind of power. Offering me four apples and one pear isn't giving me more agency than offering me a single pear, if all I want is pears!