> Is this really the "official" reason, or your interpretation?

Stanford did reach out to the American synth manufacturers looking for a company to license the patent. If I recall correctly, the problem wasn't that they didn't get it, the problem was that they weren't interested in, or capable of, building digital synths^1. At that time manufacturing LSI chips for synthesisers was absolutely cutting edge, and Yamaha had taken a major gamble in developing their in-house semiconductor manufacturing capability. Because of that, Yamaha were pretty much the only synth company capable of doing it.

1: The prototypes for the first-generation Yamaha FM synths were built with discrete components, and were enormous. It's understandable that it wouldn't be economical unless you could manufacture your own LSI chips.