Revealing mistake: As the first chopper lands on the pad, you can see that the pilot is Cowboy, a clear reuse of the shot from the episode "Cowboy"
Visible crew/equipment: As The Bus nears Rainbow Bridge, there is a reflection of either the camera vehicle or equipment in the lower right side of the windshield.
Answer: Even an Army field hospital would have SOME weapons for self-defense if the enemy attacked. What's meant is that it's not a combat outfit, and therefore wouldn't have enough weapons to actually attack anyone. (It's also mentioned a few times that the Officer of the Watch is supposed to carry a pistol at all times, but Hawkeye refuses to do it when he's in charge and Colonels Blake and Potter don't enforce the rule).
Captain Defenestrator