Here "far from it" is an idiom that refers to that it is not true. Alternate translation: "This is certainly not true" (See: [[rc://en/ta/man/jit/figs-idiom]])
Jonathan uses this question to emphasize that Saul would have told him if he planned to kill David. This rhetorical question can be translated as a statement. Alternate translation: "My father has no reason to hide this thing from me!" or "If this thing were true, my father would certainly let me know!" (See: [[rc://en/ta/man/jit/figs-rquestion]])