Information is how players engage with a game. Any game.
It doesn't mean that there cannot be hidden information. The board game Clue came up in one of the recent threads on the topic... the whole point of play is the find out what's hidden in the envelope. Players exercise their agency by making the moves that the game allows to declare their theories, which help them eliminate suspects, weapons, and rooms until they believe that they know what's in the envelope.
They can fail. They can act on less information than is ideal. What doesn't happen is that information that could be available to them is instead not provided to them. There isn't a GM who says "Sorry, you can't go into the Conservatory right now... Miss Scarlet and Professor Plum are inside and have locked the doors" or "Sorry, Mrs. White doesn't answer your summons to the Study, so you can't cross her off your list this round."
There's no GM judgment that potentially blocks information. That you consider this "meta" in some way is telling. You're too focused on the make believe of play rather than the play of the game.
As for my "agenda", I've said this is simply my observation based on my experiences with GMing and playing this type of game, and the descriptions offered here by others.
I don't think I'm "redefining" agency to match any preference of mine so much as you've already limited it to match your preference. I'm looking at player agency as the agency that a player of a game has. I don't see the need to limit it in some artificial way as you have done.
I don't think sandbox play is just a choice between railroads. I think it does allow for choice. I do think it's very focused on GM-authored material... so I think that although it allows for player freedom to engage with the material how they like (within the constraints of the fictional world of play), it doesn't go as far as many claim when it comes to player-focus.
And yes, I'm viewing any mention of "realism" as "consistent and plausible", meaning that there is some sort of logic applied to the events of play determined by the GM.
No, there's only one type of agency. You and others don't care about player agency beyond what the character says and does. Hence, you prioritize that over player agency, with that being something like "immersion" or something similar.
I don't view it as the gold standard. I am not making any value judgment. I've pointed out many times that I play and run these sorts of games and have a perfectly good time doing so. There is nothing wrong with them in any way. My statements are made as neutrally as I can manage. I enjoy these games, I also enjoy Narrativist or Story Now games.
I absolutely understand why people limit agency to only that of what the characters know and can do. It's a very standard approach to play.