China can launch a lot more than a dozen balloons, perhaps thousands if convincing decoy payloads can be made. The A-12 was intended to counter high-value targets in a limited number, not low-value targets en-masse.

I can't rule out that Issacman does have this mission profile in mind, but it seems contrived. Whatever weapon the SR-71 could carry to-altitude could be be done cheaper, more numerously and probably more quickly too with F-15s. Refurbishing a fleet of SR-71s and a fleet of KC-135Qs would be a lot of work for an extremely limited capability.