The fact that US develops X-47B seems to suggest that DF-21D is a credible threat to them. If DF-21D is not a real threat, why station your carrier 3000 miles away, and dvelop carrier-borne stealth UCAV?
I think this is an indirect confirmation that they know better how effective DF-21D is.
Well I would actually question that, because the only report of DF-21D is coming from US, China have not even confirm the existence of such missile, so we have no confirmation of any performance of this missile, and if US thinks it is a threat they do what they do, but it does not mean anything officially because we don't know anything. But to me it seems so far DF-21D is already making it is impact psychological rather than on the battlefield, and let's hope it stays that way.
Secondly, UCAV is the next evolution of toys being used on the battlefield, with or without China US is going to develop it anyway. Simply because drones offers so many potential advantages over manned platforms. So you cannot say X-47B's sole existence is China.