The reason you see different sources saying different things about DFARS and NIST SP 800-171 is because, while they are different, sort of, they also have the same guidelines as far as cybersecurity is concerned.
Confused? Well that is completely understandable, seeing as how it is confusing!
You see, DFARS 252.204-7012 is a supplement of the FAR act which governs "acquisitions" (aka contractors). DFARS 252.204-7012 is a supplement for Department of Defense contractors. It gives a lot of rules, but inside those