This seems like an odd version of the story. My understanding is Boeing designed lots of military aircraft, not all reached production. The 747 is the result of some of that design work.
"Time and time again there appears in print the logical but false assumption that Boeing took its losing military C-5 bid and revamped it as the commercial 747. In fact, the 747 would be an entirely original design that owes nothing to the C-5."
That said, in the same chapter he talks about how GE developed a high-bypass turbofan engine for the C-5 and it was only because they had such an engine that the 747 became possible.
But really my only point is that you should read the book if you're interested.
Many engine manufacturers were experimenting with high-bypass ratios; Honeywell, Pratt & Whitney, Rolls-Royce. GE was just first to production. If they hadn’t done it, someone else would have.
The B-747 went through a similar process. Boeing was proposing a big cargo aircraft to the USAF (the CX-HLS), but that was never built. Lockheed got the C-5 contract instead, which satisfied the USAF's need for a really big cargo plane. So the B-747 was built as a commercial plane, mostly to Pan Am's requirements.
Military-civilian commonality was mostly wishful thinking at the management levels, as it turned out.