Strange nods. He knows about this item, though he's not entirely certain what said item is. Just that it's big, it's important, the Rose Queen wants it, and this other warlock who he asked Childermass to learn about also wants it.

"You do know that once the defenses are lowered, it'll be a bloodbath to get that item, right? If you're smart, you'll get out of the way."

And if Ash doesn't know that much well hey, Strange doesn't mind giving up that tiny bit of information for free.