> NASA aren't rocket guys.
This isn’t true. NASA engineers own the design and engineering of the SLS rocket. No doubt some of the detailed engineering work is given to contractors, as is the manufacturing, but NASA’s “rocket guys” own the overall design. (And of course a lot of the big picture was dictated by Congress-still, there’s a lot of engineering decisions left in the middle between what Congress dictates and what is left up to the contractors.)
NASA has continually had rocket engineers going back to not long after its founding, when Wernher von Braun and his team transferred over from the US Army - through Mercury and Gemini, to Apollo’s Saturn V, the Space Shuttle, the abortive Constellation program, and now SLS. Part of the argument that SLS supporters make for not canceling it is that it gives NASA’s rocket engineers work to do and helps keep that knowledge in-house
NASA’s engineers also provide technical advice to commercial launch vehicle and engine vendors such as SpaceX, Blue Origin, ULA, etc, and they also have an important role to play in NASA’s verification of the commercial vendors’ technical analyses to demonstrate their launch vehicles are safe and reliable. But double-checking the work of someone else isn’t the same thing as owning your own launch vehicle design. Now, personally, I don’t agree with the argument-it isn’t worth spending billions for NASA to retain their own “full service” rocket engineering skill base when the commercial skill base in that area is now just as good, arguably even better - but that’s the argument they make