That's if anything an argument for a Saturn V carrying some platform that would look like the flight deck / cargo bay / Canadarm part of the Space Shuttle to orbit to help with in-orbit assembly.

Sure, you might need some such platform to construct something in orbit. That's basically a space forklift with thrusters.

It isn't an argument for Space Shuttle system as a whole, i.e. propulsion system, heat shield, solid rocket boosters & external tank, being needed.

For what it cost to launch the shuttle such a platform could have been entirely expendable with astronauts returning in capsules, and they'd still have saved money, and been able to take up 3-4x the amount of cargo up on each trip.