True, but still there's no answer for the US Navy to deal effectively against the PLAN DF-21D ASBM.
Actually the threat of the DF-21D is being dealt with on several very effective levels::
1) First, is discovering what on earth, if anything, it is currently capable of, or may some day be capable of. To this day there has been no life-fire, full-up, end to end operational test of the DF-21D against any moving (much less maneuvering) vessel at sea. Not one. Unil that occurs successfully, and on multiple occasions proving its effectiveness...it remains so much talk. Now, the US is taking the development of such a system seriously and projecting what it might be capable of (and doing so very liberally) in any case. To that end:
2) More and more AEGIS vessels are equipped with Ballistic Missile Defense capabilities and missiles. A system which has been, and continues to be, live ire tested, very rigorously.
3) The US is in the operational test pase of a new anti-missile defense chaff cloud made up of stealth filaments for its ship to get behind.
4) The US has very strong passive and active electronics counter measures, both ship borne and air borne to fry the electronics of incoming missiles.
5) The US will soon deploy rail gun and then laser anti-missile defenses on its Capitol vessels.
6) The US has some of the best CIWS systems, both missile and gun systems available.
7) The US has developed, and continues to develop the capability to attack the anti-access systems at all levels (meaning the surveillance, detection, acquisition, targeting, firing, and delivery systems that go into a complete system like the DF-21D) from outside of the range of those systems. Defeating or seriously degrading any one of those portions of the combined system is an effective mission kill of the system
So, the US is very well prepared to challenge and defeat such a system should it become necessary.
My own thoughts are these. Until the PRC actually conducts several full-up, live-fire tests of such a syste against targets at sea, proving its effectiveness...I consider that effort as much a Sun Tsu disinformation, mis-direction, and calculated bluff program as it is an actual, operational system.
Is the PRC working on it? Yes, absolutely. Is it a capable, operational, and fully tested system at this point? IMHO...hardly.
But the US is prepared for it or other such threats in any case.
In the mean time, except for such a system being used in concert with the activities of the PLAN carrier in combating othe vessels or CSGs, the DF-21D is really off topic here.