Even if the GM never touches the timetable, they could (VERY much non-exhaustive list):
- Reveal or resolve a conflict within the enemy ranks, to slow down or speed up their efforts
- Have an enemy act in a rash manner which imperils their plans, or learn more from a previous mistake, enhancing those plans
- Establish a distraction or complication for the enemies, or establish a recent breakthrough that overcame an existing one
- Reveal a convenient source or location which is of particular use to one side or the other (or both!)
- Decide that someone useful has been subjected to, or liberated from, mind control
- Replace an ally with a doppelganger, have a spy show up for one side or the other, various other personnel things
- Reveal a sudden windfall, perhaps precipitated by an action on one side or the other
Etc., etc., etc.
The "traditional GM", in having
absolute and secret control over 100% of facts in the world, can do any of these things at any time for any reason, and can even delay actually developing a justification until after the PCs have done enough work to learn what the justification might be (though it is, as noted, part of the "gentleperson's agreement" that the GM should not delay that justification-development unless they have no other choice).
By having such control--specifically being both absolute and secret--timeline concerns simply cannot actually limit the GM's behavior. They are, always, capable of simply doing a bit of work to develop a reason why the timetable
needs to change because what stuff is in the world has changed. Indeed, it would almost surely be utterly unacceptable to you, and Micah, and Lanefan, and most other "traditional-GM"-preferring, sim-focused players for it to be the case that a previously-known timetable could
ever be more important than developments in the world that would contradict that timeline. That would mean that a metagame construct--a timeline--took precedence over information from the fictional world, such as any of the developments in the bullet points above.