I agree that it increases the chance of getting through, as I believe does the "low flying" Russian Topol, and allows a credible deterrent with fewer total missiles.
While you don't get as much time to see it coming over the horizon, it is easier to see it: it'll be much hotter than a higher vehicle, and any decoys need to be similar in weight and size, not just floating.
I suspect the best place to stop it would be not long before it arrives, having been tracked from where it departed.
It would require some upgrading of a credible defense, and so yes, I guess it is a threat - although a hypothetical one - since China already has enough missiles to overwhelm the existing US defense system.