Technically, dehiscence is when any kind of fruit splits open to release seeds, so not just milkweeds. Also technically, what you’re calling Centaurea seeds are really fruit – dry ovaries tightly covering a single seed. They are indehiscent, since they don’t split open, but I don’t know of a technical term for their particular dispersal method. Botanists do love their terminology, though, so someone may have come up with something.