You are assuming. Nothing in that section says that. Further, even if there was another test of player established motivation to get them there, it's still a test of player established motivation. And...........................they can still say no and go somewhere else.
That section doesn't tell the DM to try over and over until they get there. That's you placing your bias into the mix. That section only gives examples of testing their motivations through DM established content. It doesn't tell them to railroad the players.
I don't think I'm assuming a whole lot. I'm basing this off a reading of the actual text. Here's what it says (highlights mine):
"Motivation is what drives the adventure- it's what gets the PCs involved in whatever you have designed for them to do. If the PCs aren't motivated, they won't do what you want them to, and all your work will be wasted."
I'm not sure how you can read that passage, or any of what follows, and read it as the GM testing the PCs' motivations. It's about using their motivations as a means of getting them to engage with the pre-authored material the GM has made. Why else point out that all your work would be wasted if they don't engage?
If you construct an understanding of agency that specifically precludes the GM from creating content, then it's pretty trivial to claim the players don't have any.
How much must the GM adjust their content to shift in response to player/character motivation for it to count? Moreover, does it provide more agency when a payer chooses not to engage with something, or when they're never offered a chance to engage in the first place?
I mean, the fact that you describe it as "their (the GM's) content" says a lot. It means that the GM has prepared material and now needs to figure out how to get the PCs involved.
Per the passage, it seems that you should be ready to do what you need to in order to get the PCs motivated to engage with the scenario. How much? I don't know... I suppose it would depend on the GM and how easily they could come up with something else, or how comfortable they are GMing by improv.
What I'm saying is... What's the functional difference between, "This PC is a wizard looking for the secret to immortality, let me write up a wizard's tower" and "I wrote up a wizard's tower and l can tie it in the the PC wizard's desire for immortality?"
The DM's motivation is different, but both instances result in a wizard's tower that ties in to the player's stated priorities. The result is the same.
Who's desire is it to search the tower? Or more accurately, whose desire does the tower represent? Why did the GM come up with the wizard's tower?
Except we don't know that, do we?
For all we know, if Mialee hadn't died the PCs might have gone nd done something different and never known about the Cleric and the wererats.
Your take here assumes the GM is locked in to running what's prepped, which isn't always the case.
It really doesn't. The opening passage, quoted and highlighted above, makes it pretty clear that the GM is meant to find a way to get the players to engage with his material. I don't even see how that can be in doubt.
Is it possible that instead of the temple and wererats, the GM has some other material prepped that he can more easily motivate the PCs to engage with? Sure. But does that really make a difference?