So using tense muscles as armour? That’s a very strange strategy since muscles are richly interacted by sensory (pain) nerves (Group IIIs) and highly vascular.Uechi ryu has the remnants of Chinese iron shirt training. Kanbun Uechi was training in China not long after the boxer rebellion and there is a historic link to another rebellion too but that's a long story. Needles to say it's iron shirt training. I would guess the same for Goju but I don't have as much history on that.
With Uechi the goal is to have the ability to take hits and fluff them off like it's nothing. You do need mobility so you can't just stand there with all your muscles locked down. Thus it's often said in Uechi and Goju , "half hard -half soft.
I do belive Goju had the same end goal as Uechi. My speculation is that Goju would start off very hard then reduce the tension as the student became more proficient. But that was lost over time. Where Uech didn't go-to that extreme.
Dynamic tension is a key component in iron shirt training. I think Uechi and Goju just had different starting points. Then lost a lot of the training when it left China.