I was also a bit confused by this. For fine motor control, right handers will use the right hand, not the left. The hypothesis is garbage.

You two are right. Reading it again I think I corrected the mixed up hands in my head. While they say the chisel is in the right and the hammer in the left, everything else depicted paints the opposite picture in my head.

If you hold the hammer in the right and the chisel in the left, you swing the hammer from the right to the left with the chisel tilted to the right. Thus the natural writing direction (outward from the swing) would be right to left. Swinging your hammer from the right and moving to the left would mean writing in the opposite direction of the force used to write single letters.

So, if the hammer is in the right hand and chisel in the left means a right-to-left motion and makes a left-to-right motion counterintuitive.

I would think that the "fine motor control" would be in holding the chisel, not the hammer. But you find the same pattern for violin or guitar -- "right-handed" playing has the right hand holding the bow or strumming, while the left hand does the fingerings. The fingering is absolutely more difficult; but I think (from my own experience) the main thing is which hand is in control: The bow and the strumming causes the sound to come out, so that's what the right hand wants to do. That's how I feel about using a chisel too.

You need to put chisel in a right place, than strike it with a hammer. So once the chisel is in place, the only thing the left hand does is holding chisel, which is not hard at all.

Think of it as a nail: if you need to hammer in a nail in exactly the right place, you put the nail into exact position holding it with a left hand, then strike it with force using hammer in the right hand. And if you ever did this you know that the tricky part is hitting the nail the right way, so you don't bend it.