And why couldn't, say, the Inca draw on the Wari, who could draw on the Moche, who could draw on the Chavin, who date back even before the Greeks?
The problem with claiming the utility of writing in developing military tactics is that Western Europe doesn't have a tradition of discussing military tactics in written texts until the Early Modern period. There's nothing like Sun Tzu's Art of War that keeps getting passed down and talked about; any transmission of tactics is going to happen via practical experience in a kind of apprenticeship--which is exactly the same method of transmission an illiterate society is going to do for military tactics.
Or you could do what the Aztecs did and send all of your boys (rich or poor) to school to learn how to become warriors, come to think of it.
If you've got evidence that the Inca military had organized tactics, like units, feinting maneuvers, flanking attacks, procedures for taking fortified positions, covering fire, strategic retreat, breastworks, defense in depth, etc., I'm interested.
We do know the Inca had no plan for when their leader was captured but not killed. But Pizarro knew about that one, and that's how he defeated an empire. Disrupting the enemy's command and control is a well-understood technique in Western military tradition.
The way they operated was clear evidence of military sophistication.