A Former Weakness
Amery was in the same exact state that Leonel remembered him being in: sweating buckets and swinging the very same wooden sword. In fact, the only thing that had changed was that he was using a different hand, but Leonel was pretty sure that was a coincidence.
Amery had always been a dual sword wielder, so it wasn't a surprise that he would use both hands. In fact, part of the reason for that was because his Innate Nodes were in his hands.
He probably wasn't born ambidextrous, but had instead learned it. That said, it was clear that he had a proclivity toward it nonetheless.
The reason it would be easier for him to learn to use his off-hand compared to others was precisely because of that Innate Node in his left hand. It made certain that the Nodal Pathways in his arm were just as robust as the Nodal Pathways in his dominant arm. Thus, so long as he trained his muscle memory, the skill would come naturally.