@Crimson Longinus
System's Say isn't just about dice mechanics. It can just as well relate to the sorts of judgements and the basis of the judgements the GM and/or other players are expected to make. This matters for agency because my ability to anticipate, determine and constrain the sorts of things that can and will be said in follow up is a significant component of agency.
If I know the GM should be making decisions solely based on fictional positioning and their understanding of the NPCs involved it then becomes worthwhile to engage in exploratory play to discover information to leverage.
If I know what the consequence space and odds of success with Go Aggro are than I can consider that when choosing to do something in the fiction that would count as Go Aggro.
Knowing position and effect, stating my intent and knowing my action rating in Blades in the Dark provides me with a whole host of factors that influence my decision making and allow me to make decisions that should forward my aims.
When it comes to all sorts of math rocks it's less about the dice than knowing how those dice can impact what happens next and finding ways to influence what dice get rolled, what that target number will be, how those dice get modified and what happens as a result of those dice rolls.
It's not about dice versus GM decisions. It's about being able to foresee the impact of one's actions in order to make decisions that impact what happens next in predictable ways. There will be unanticipated consequences along the way, but a substantial component of agency involves the ability to anticipate and impact the chances and nature of success and failure.
A fundamental part of agency is that in exercising it you impact the agency of others. Once you have acted everyone else in the conversation is constrained by what you have added to the conversation. They must have a regard for it, and it will dictate and constrain the types of things they are allowed to add to the conversation according to the principles / agreements / system that dictate the terms of the conversation.