What if they wanted the attack to happen? Or someone else did and simply ignored her not because they didn't believe DIMA, but because they felt this would result in the outcome they wanted or that the SDI would make what the Chinese had planned, moot.
Hell, I wonder if the Chinese managed to slip a few backpack nukes into Fallout America before the bombs fell?
Thing is that the SDI system relies on reaction time, and if games like
Children of a Dead Earth and
GURPS told me anything, to get a
useful SDI laser (i.e. a long-range laser capable of hitting small targets at incredible distances), it'd be a
heavy monster meaning that you need either a massive, complex mechanical setup or a massive,
power-hungry electromotive system to move it quickly (basically, you'll need a fission plant
just to move the entire array). If these subs came in a few kilometers from shore, then the arc of the SLBMs would be incredibly flat and your interception window is incredibly narrow.
Remember that scene in
Hunt for the Red October where the one tech that Jack trusted was given photos of the titular submarine's propulsion system? Those Chinese stealth SSBNs could pull that off.
So, basically, imagine playing
Missile Command but the missiles appear within seconds of your cities, and you'll get the idea of what the US SDI network was dealing with. I wouldn't be surprised that those stealth SSBN's job was to knock out as many SDI sites as possible in the first salvo.