To me, @Hussar seems perfectly aware of that difference, and is not confusing it. He seems to be pointing out that choosing from stuff that the GM presents to the players is still about the GM's idea of how things in the setting, and hence the game, hang together.You are confusing player input on setting with player choice among things in the setting.
I don't see @Hussar having asserted that anyone is being railroaded on a single adventure.The players could have gone to someone or someplace else other than the sage. They could could have completely shifted focus to another thing (I.e. forget about the spelljammer let’s go see if the folks at Dragon Hall are hiring guards). That the GM is in charge of setting content isn’t relevant here. And you are still wrong IMO about how much player choice and action shapes what the GM does with the setting but that is a whole separate conversation about GM authority than whether or not the players are being railroaded on a single adventure or are free to choose what they want to spend time pursuing
What he is saying, as I read it - and in response to another poster's example of the Spelljammer - is that if the players want to pursue the goal of obtaining a Spelljammer, then they have to go through the steps authored into the setting by the GM. That is, first they have to find someone (eg the sage) who can tell them where a Spelljammer might be found; then they have to find out how to get to that place; then they have to go there; etc. The players have to proceed through a series of steps, or events, that the GM has authored.
The fact that the players might change their focus to some other item on the GM's menu (to continue @Hussar's metaphor) doesn't change the fact that it is the GM's menu.