Any test mixes signal with noise (Maybe that superstar candidate is just awful at interviews? Maybe the super lazy coder puts their laziness aside only for the interview day?)

Combining many noisy factors into one, with the correct weighting, will always, on average, give better results than a single test.

That means that as soon as DNA tests give even a weak prediction of 'iq' or 'leadership abilities', companies will integrate them into hiring processes if legal and cheap.