Albert Barnes Commentary 1 Samuel 21:11

"And the servants of Achish said unto him, Is not this David the king of the land? did they not sing one to another of him in dances, saying, Saul hath slain his thousands, And David his ten thousands?" — 1 Samuel 21:11 (ASV)

The king of the land - The Philistines gave him the title which their own lords bore.