Supposedly, it’s maneuverable, not ballistic. We have had maneuverable reentry vehicles (MARVs) for many decades, going back at least to the Pershing II, with it’s mach 8 MARV. A high-G maneuvering vehicle traveling at mach 20 would certainly be a very, very, challenging target. That said, Russia has had serious problems with previous supersonic missiles suffering catastrophic failure when attempting the high-G maneuvers required to avoid modern air defenses. Historically, Russia has always over stated the capabilities of their weapons, while we tend to understate ours. If we decided that we didn’t want this ability, it’s likely there was an unacceptable trade off in reliability and accuracy. Or we decided on an alternative technology, like stealth or decoys, to ensure a kill.
I think you are correct, especially the part about accuracy. If there were no winds or the winds were predictable then maneuvering would be more precise. Note that although the atmosphere is thin at 50 miles or so it may be moving fairly fast which can throw off a calculated post maneuver trajectory rather significantly. Unless you want to put in a much larger yield to cover that type of slop you may miss by enough to fail to destroy a hard target.
[A high-G maneuvering vehicle traveling at mach 20 would certainly be a very, very, challenging target.]