It is an affront to our national intellect to dignify this discussion by even suggesting that there are two competing “theories.”
There is what is unlawful discrimination and it IS disparate treatment. That’s not a theory. It is reality.
Then there is the tactic of alleging disparate impact. Also not a theory —strictly a tactic.
The goal of the tactic: to abuse well intentioned laws against discrimination (see above) by warping them into tools for social engineering wielded by the legions of unaccountable Pinkos that infest the Federal bureaucracies.
It’s what happens when the Left gets control of the language.