The missile in question is obviously not the Titan II that was used for portraying it. The first stage has a futuristic single, broad nozzle, similar to the one that propels the warp upper stage at sublight. And the spatial version of this engine apparently has the ability of getting the warp stage back to Earth from a considerable translunar distance in a matter of hours rather than days.
This is simply 21st century treknology at work. The engine simply is that good. No, this is not surplus from 1970 - it is surplus from something like 2030-2050, and only incidentally looks like 1970s tech. And what else could
it look like? A rocket is a rocket - you don't design one to look like a vagina just because you are wary of being phallic. And USAF would apparently cling on to its traditional colors till WWIII and perhaps beyond.
Why give the lower stage of an ICBM a SSTO capability? Many advantages there: launching to orbit at one point and then firing down from there at another moment will surprise the enemy, and might also make him hesitate at the moment of launch. Targets could be altered in mid-flight, and off-planet targets such as freely orbiting space stations or maneuvering orbital combat platforms could also be hit.