They don't have to tell anybody, if it's what she wants. Draco's only friends in the whole world apparently already know, so he's no longer fussed on that front. But if she wants it a secret... he can do that. He will do that, if it's what Hermione wants. But he's going to propose that they definitely not share. He'll get it out in the open this time. He thinks they probably arrived at that conclusion anyway, before he carried on with the physical part of his apology, but he'll make it a spoken thing.