"They eat." he responds with emphasis, trying to encompass the whole itching, chittering, gnawing quality of it. "When he had his Noah they wouldn't touch him, just lived inside his body. Without it... well. Only one way out, isn't there?"
Never mind that he helped Tyki with that the last time, in a decisive bite-come-improvised-tracheotomy, but Jack doesn't need to know that. Spike is so goddamn sure that Tyki won't actually do anything and this is his attempt at bringing that point home, to calm people down but just as much to give Tyki some space. Clashing right now would be cruel and unnecessary — who would've ever guessed that he'd resort to diplomacy?
"So that's where we're at. He's just a bloke, Jack. Not in any state to harm a flea, let alone kill anyone."