A 'know-it-all' is someone who THINKS that they do know everything. And, while they may know a great deal, those around them often can discern where they are lacking in knowledge, or in understanding of something.
I could imagine the sentence reading: "He is someone who knows-it-all, but doesn't know (anything) about love."
AHD: know-it-all (n½"¹t-ôl") n. Informal. One who claims to know everything and rejects advice or information from others