C. F. Keil and Franz Delitzsch on 2 Kings 18:20

2Ki 18:20 “Thou sayest ... it is only a lip-word...: counsel and might for battle;” i.e., if thou speakest of counsel and might for battle, that is only שׂפתים דּבר, a word that merely comes from the lips, not from the heart, the seat of the understanding, i.e., a foolish and inconsiderate saying (cf. Pro 14:23; Job 11:2). - עמרתּ is to be preferred to the אמרתּי of Isaiah as the more original of the two. עתּה, now, sc. we will see on whom thou didst rely, when thou didst rebel against me.