Hopefully it's more like "what kind of safe do you keep the keys to your other safe in?"

I think the point is in relying on something that is fundamentally secure, not secure because it is obscure. If I print my private key on a piece of paper and pin it up by my desk that's something I shouldn't tell people, but if I have it locked in an unspecified safe deposit box, there isn't much harm in describing what the deposit box is made out of.

It does make you less secure, but a good security plan starts from assuming your adversary knew all that sort of thing in the first place.