Do players have event timing in D&D? I'd say absolutely not -- this is solely the realm of the GM. So, yes, I agree, there is a loss of agency when you move that to a place of less control -- the GM has less agency. Perhaps overall agency, GM+players, decreases -- I'm hard pressed to be able to make that argument either way -- it's close, at least. But, the question in this thread is about player agency, and it's clear that this approach does enable player agency more, because while they don't have control over event timing, so to speak, they have a lot more input on exactly what and how that event will resolve and the GM cannot block or say no.
It, to me, is a very simple point. If the GM cannot say no, and is bound by the result, then the player has more agency than in a case where the GM can say no and is not bound. Clearly, though, the GM has less agency in the latter.