i don't think he needed to make that promise - i'm not sure though. if more email had been found that could be linked to that previous investigation of course that case could've been reopened. but ultimately that investigation was part of a case the FBI had developed in order to turn over the result to DOJ. it wasn't a Congressional investigation. well, Congress investigated the FBI's recommendation to DOJ but Congress wasn't investigating Clinton's email, per se. so i think Comey could've testified about that recommendation and left it alone. but he added that little nugget and so here we are.