As the Lord liveth - The swearing by Yahweh seems strange in the mouth of a Philistine. But probably not the very words, but only the sense of this and such like speeches, is preserved.
Related Commentaries of 1 Samuel 29:6
1 Samuel 29:6
6 Then Achish called David, and said unto him, Surely, as the LORD liveth, thou hast been upright, and thy going out and thy coming in with me in the host is good in my sight: for I have not found evil in thee since the day of thy coming unto me unto this day: nevertheless the lordsa favour thee not.