I think you have a good theory but I've just had a beer so I've dreamed up another:
The engine is aligned in the frame correctly by the steel sleeves inside the iso-rubbers.
The rubbers soak up vibration but they would allow the engine to twist slightly against the chain pull on hard acceleration.
So Honda have popped in a little strap that stops the motor twisting left or right.
Notice how the strap can move up and down but not left and right when tightened up.
Hey, it could just be beer talk.

Anyway, let's crawl back to the swinging arm spacer issue…