I’ve never actually played with OS settings for this and the mice we do have here are contoured for use in the right-hand. This isn’t such a big deal because the left-handed users here either have touch-pads on their laptops (the MacBooks), use a Microsoft Surface, or are right-handed gamers.
If, as you say, left-handed mice can be setup to use the left index finger on the mouse as “primary click”, then yeah - there isn’t really a big deal between the two in regards to left/right handedness.
As for Ton, his stated position didn’t have anything to do with that (though perhaps some of his unstated position did). For him, it was supposed to be about evening out the clicks between the two fingers on the (I can only assume wishful) assumption the 3D cursor would be as equally important to the workflow as selection would be. That’s really never been the case.