"competency only found in the upper tranches (say top 1/3) of students in high school.
yet not a single one of these things has the slightest to do with technical aptitude"
?? Aptitude is definitely correlated with academic performance. There is no debate there.
I'm sure on your journey you've taken enough stats to grasp that your personal anecdotal experience doesn't count for that much? I mean, being last place in school, absent all the time ... would it be reasonable for you, the Uni, or anyone to believe that you were 'well suited' to Engineering, or at least more suited than those with good grades, GPA yada yada? It's great you did well, but you must agree that wasn't likely.
In the aggregate, both GPA and SAT are highly correlated with Academic performance in University, so you're arguing against the wind here. [1]
(Do I really even need to provide a data point on this?)
And you must know that being 'Last Place' in High School would preclude most kids from even being accepted to Eng. programs, let alone Uni.
Students who do poorly in High School generally won't succeed in Eng. programs - let alone be accepted in the first place.
Everyone knows this, and it's why they use Grades and SAT as a primary means of admission.
And FYI raw aptitude can't be taught but all sorts of other things can i.e. having basic mathematical literacy and just 'keeping up' from grades 1-9, means that kids have the confidence and opportunity to participate in 'STEM' things which they would be blocked from doing otherwise. Peers, Mentors, points of Inspiration also give kids the extra energy and ethos to work through the issues they might not otherwise care about, in effect, there's a lot of 'passion' hidden inside 'aptitude'.
[1] https://www.forbes.com/sites/prestoncooper2/2018/06/11/what-...