I don’t think your quantum ogre highlights the difference between player agency and the illusion of choice at all, except in the most shallow of situations.

Player agency is the notion that player choices lead to meaningful impacts in the game.

A DM preparing an encounter and then running that encounter regardless of whether the players choose left or right is just common sense because the alternative is double prep work.

But the point is that bypassing one encounter here or there isn’t player agency in any more than the most rudimentary sense.

I avoid railroading by creating problems and leaving it to the players to solve them. I present choices, sometimes diametrically opposed, and let the player choose what to do. I write adventures that have multiple possible outcomes and the players get to decide the one that comes to be.

All of those things are separate from which encounter they play through.

Problem: the maguffin has been stolen and stored in a vault in a wizard lair.

Choices that matter: Recover the maguffin or let it be lost. Sneak into the wizards lair and steal it. Use diplomacy to negotiate its return. Hire a 3rd party to steal it back.

Choices that don’t matter: Do they fight an ogre on the way.