I'm not an attorney (far from it) but I share your sentiment in that AA and legacy admissions aren't the same when viewed in context of the 14th amendment, because legacy admissions are race neutral on the surface (as opposed to AA). Now admittedly I don't know how the court rules based on disparate impact because at least in its current context legacy admits benefit more white people. But I don't know if that's something the Supreme Court rules on.
I think disparate impact is more of a political than legal concern (at least today). Discrimination based on race is clearly an intended goal of the 14th amendment. Discrimination based on legacy, not so much. College admissions criteria also include geography, ability to overcome adversity (leading to professional essay writers), first generation college students...How many of these are protected under the equal protection clause? Seems like we can get too broad with equal protection.

