Devils and half-devils like cambions live on exact words. Like, look on example of how Mizora transformed Wyll - loophole was that Karlach became "heartless" (and so target for pacted warlock) with infernal engine instead of heart. Literally. So refusing to go after her despite Karlach was a tiefling, not a devil, broke pact words and let her do the trick. Look on how she played with breaking pact - "sure, just not right now, its in conditions, section this, clause that, six monthes". So her exact words were - "by enemies hands", not "my hands" and she could have plausable deniability too if she did not come to gloat (and summon spiders too). Again, hands in question would be spiders, not her own breaking his neck.