Didn't Apollo 11 land on the moon using a rocket, then take off from the moon again, back in the 1960s?
Not exactly a tech demo. And the Apollo missions had the additional challenges of being crewed, and targeting an atmosphere and gravity they couldn't reproduce on earth for test purposes.
The SpaceX stuff is neat though, compared to the defence industry clowns they're competing with.
To do what Apollo 11 did without discarding the boosters you also need orbital refueling and probably rapid turnaround (or a huge inventory of boosters), which SpaceX plans to develop next. Awesome stuff.
You know, this bit https://en.wikipedia.org/wiki/Apollo_Lunar_Module that descended to the moon, landed, some guys walked out and grabbed some moon rocks, then they took off again and made it back to earth.
That's a rocket-propelled space vehicle gently landing tail first, and ready for immediately reuse.
Given that it clearly had been done, I doubt anyone who knew what they were talking about was telling Musk it couldn't be done.