“So, both are swinging a sword and holding a shield, but in mirror image? Wow, that does sound like one side would have an advantage. (not)”
In a complete vacuum correct.
But in such an encounter, the lefty would have fought (or practiced against) his mirror image many times, the righty much less so, or not at all.
Advantage lefty.
In a first encounter, a lefty will not have done that. No advantage, no complete vacuum either. Real life.