I did not know about this, thank you very much! It's technically very interesting. Moving nozzle extension has been demonstrated in bench tests with running (and gimbaling) engines, and in flight with stopped engines (Delta IV's second stage RL-10).
The dense fuels of Titan allow high mass ratios to be achieved, and it's long been known that the ordinary Titan II or III or IV stages already had SSTO performance in that way.

In the Soviet Union, Proton went the route of storable fuels and high chamber pressures (with staged combustion) back then. USA has only fielded one high pressure engine, the SSME.

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