I'm not trying to detract from the monumental achievement SpaceX's engineers have made with the development of the Raptor, but that development work didn't take

I'm not trying to detract from the monumental achievement SpaceX's engineers have made with the development of the Raptor, but that development work didn't take place in a vacuum. Unfortunately, I don't have a credible source for this, but I have seen multiple reports that the Raptor program was explicitly a continuation of the IPD program, with SpaceX receiving technical data, hardware, and even hiring engineers who worked on it. That the IPD was hydrolox and didn't proceed to the point of a full engine, let alone a flight is besides the point. What you said was: "They kind of did invent full-flow engines though", which is completely false.