Said argument also assumes that despite the few times we've seen Cowl attack Harry, he wasn't REALLY trying to kill him, otherwise he'd be committing suicide. And so Cowl seems like he's been trying pretty hard to do so.
Also, the multiple people making comments about collecting bottle caps, and I think Mab and Harry had a phrase they shared. This all comes from the mind of one person, who writes everyone's dialogue. He's got only a finite number of phrases he can draw on, there's bound to be repeats.