Why are you assuming situs inversus, which occurs in species with no handedness (or, indeed, hands) came after handedness?

The argument is that the selection bias was towards precision and the hypothesis was that precision is influenced by heart position (which is, still, in the middle in humans)… individuals with situs inversus would be more precise in the left hand, thus if the causal hypothesis is correct AND the argument holds then there should be a selection bias that would result in a correlation between situs inversus presence and left-handedness.

In the end I don’t believe either the argument or the hypothesis hold even as much water as I can in either hand.