John Gill Commentary 1 Samuel 20:39

"But the lad knew not anything: only Jonathan and David knew the matter." — 1 Samuel 20:39 (ASV)

But the lad knew nothing
What was meant by shooting the arrows, and by shooting them beyond where he was, and by telling him to hurry and bring them:

only Jonathan and David knew the matter;
what was signified by them, as these were signs agreed upon between them.