One day, I hope to actually play DnD.
Having agency is based on there being meaningful consequences of your actions, not being able to declare the consequences of your actions (not the same thing as declaring the intent of your actions) or not knowing the consequences of a choice with agency when you make it, are not the same as not making the choice with agency.If the consequences of a player's declared actions are decided by someone else (eg the GM) by reference to stuff that is more-or-less outside the player's control (eg secret backstory that only the GM is privy to), then where is the agency?
In high player-agency RPGing, the players have a lot of capacity to establish what is at stake. In the OP, the players seem to have had almost no say over what is at stake.
If I attempt to lockpick open a chest it is my agency to declare that i am trying to open it, it is not determined by the hypothetical contents of it, If it is trapped, If someone sees me doing it or even if i actually succeed the check but my ability to attempt to open it and the fiction following meaningful logical consequences from my attempts to do so.
edit: the contents of the chest may be significant to agency in the scenario that they should exist but don't purely for the point of denying the player any reward/consequence for successfully picking the lock.