Landing rocket boosters happened in the 70s already, and said rocket company benefited heavily from NASA, government and defence contracts and research. If Boom would have had access to supersonic jet research from the government, along with government contracts, development or production doesn't matter, RR wouldn't have stopped the cooperation.
Edit: Reusable or VTOL rocket were tested, successfully landing, in the early 90s by McDonnel Douglas under the DC-X program. No idea where the 70s thing came from...