I think it goes to show the incredible depth of engineering in the U.S. to go from start in 2002 to humans in 2015, 13 years, with all new tools, designs, engineering and manufacture with so few people and such little monetary investment. It's astonishing...
Unfortunately, I can't find a good source for it (I remember reading an interview with Musk where he talked about it). I think everyone recognizes that they are standing on the shoulders of giants and says so frequently.
Interestingly, the Soyuz and Apollo programs, although their missions were similar, differed significantly in engineering culture. I heard a story about the Soyuz-NASA collaboration that lead up to the Apollo-Soyuz Test Project (http://en.wikipedia.org/wiki/Apollo%E2%80%93Soyuz_Test_Proje...): NASA engineers met with Soyuz engineers and asked for engineering specifications, but the Soyuz engineers couldn't give them (or maybe they just weren't very detailed) because most things were in people's heads rather than documented. The Russians were iterative and practical, and had very robust systems, but they were not nearly as formal as the NASA blokes.
In the end, I wonder if that lack of careful documentation on Soyuz' part diminished its long-term helpfulness? Or did they later record their designs more carefully? I would be curious to know. Apollo documents are apparently available for aspiring rocket builders. Hurray for openness in publicly funded research.