[dmurray]

It's assumed that a submarine can travel in the ocean for long periods without detection. I don't know whether this is still considered to be true by military strategists, but it's a large part of the motivation behind having nuclear-armed submarines in the first place.

A submarine is at least pretty stealthy, but the heat signature of a missile launch is impossible to hide.

Another way to look at it: the satellite systems described in the article (SBIRS and DSP) are optimised for early detection of a launch. So when considering the deficiencies of those systems, it's correct to consider a hypothetical adversary who optimises for the opposite.