I'd been wondering about why Taylor reacted so differently to him than to a certain other know-it-all smartass mastermind type, and I think this is a big part of it. Lisa shared Eithan's habit of acting like a jackass for her own amusement, but Taylor knew her well enough to see that behind the facade, she was burning the candle at both ends because of how seriously she took her responsibilities. With Eithan, she has no way of knowing if he refrained from telling her about the Penance prototype sooner because he knew he'd need it when a situation like this came up or because he thought preventing the whole multi-monarch battle by letting Malice use it to assassinate Shen would have been boring.