He shrugs rather than doing something like correcting or explaining. He can do more than the bubble, for sure, it's just awfully convenient. And sidestepping the explanation is no longer about secrecy here, really - more that he just still doesn't like to talk about himself and what he can do as if that's somehow important. He showed off a bit for Law for fun when Law was still a kid, but with adults it strikes him as bragging.

"Think of the space around him as his operating room. He can control how large an area that is depending on what he needs to do, but within that room he has full control, like a surgeon with all his tools and equipment. Imagine being a surgeon that, instead of having to ask for a scalpel, could just bring one to hand? That's what he's doing, it's just that he can do that with people, not just scalpels."