Not a crock. A leftie gets to practice against right handlers all the time. The other way not so much.
And in combat, the southpaw would have exactly the same setup as the right-handed majority, because that's the weapons that are around. Even in today's more enlightened world, people are still learning to write right-handed because they're more or less compelled to. In the world of ancient or medieval combat, lefties would be at a distinct disadvantage having to use stuff designed for right-handed warriors, or (and this has a natural selection angle) would avoid combat, which is why the characteristic hasn't been bred out. Mostly likely, since humans are naturally inclined to mortal struggle, they'd resort to sneaky left-handed dagger thrusts, basically becoming assassins.
Agreed.