Not to detract from most of the points you make:
I think he means FOBS more than the typical MIRVed fast-burn ICBM approach, and yes, a reasonably competent SSTO would be capable of this. Look at the mass of a typical RV to get an idea of the effective 'throw weight' that the SSTO would require for a given strike; I wouldn't be surprised if reasonably-competent college tech classes couldn't build a nav system for post-release guidance, or the analogue to a warhead bus. We can't really rely on restrictions on MARV technology, either, although that does involve 'slightly' more sophisticated materials and engineering...
You seem to be predicating some of your thinking on an assumption that the only feasible launch mode would be 'cold-war' style massive TOT salvoes following the general scenarii from mutual-assured-deterrence models. That's somewhat unlikely to be the method of attack used by "minor" nuclear powers today. You also assume that 'launch detection' would follow the same model as was used traditionally for ICBMs. While it's possible that a nation like North Korea might try to build a 'sufficiently' large number of SSTO vehicles and then launch them en masse for some matter of perceived national pride (or suicide), I'd be more concerned about things like 'terrorist diversion' of one or more vehicles (nudge, nudge, wink,wink, plausible denial anyone?). Would you order these to be shot down immediately, possibly knowing about as much as the captain of the Vincennes did about the actual threat posed by the vehicle(s)?
If you remember the 'original' concerns about FOBS, once something is on-orbit (or in a fundamentally-'orbital' trajectory with corresponding speed and momentum) most of the work needed for prompt-strike is accomplished. (And the job of rapid response, already complicated here by uncertainty about what might constitute first sign of an attack, or where the lags might be before an adequate exoatmosphere interdiction could be commanded, becomes almost intolerable...) I have always thought this was one of the principal concerns behind official Administration policy toward X-33/VentureStar cancellation "after the lesson of 9/11" and in the framework of diplomatic arms-control concerns.
I would, indeed, have concerns about the use of commercial space vehicles as potential weapons platforms (even before we start getting into Q-ship discussions...)
Just extending the picture a bit.