In the 2022 survey (I didn’t see the stat in 2023), 35% of entering freshmen did not submit scores, which is ~525 students. Half of recruited athletes did not submit scores, which is ~75 students. Therefore ~75/525 = 14% of test-optional students were recruited athletes. I wouldn’t call 14% a “large portion.” This figure would almost certainly be much smaller at Cornell because a much smaller portion of entering class is recruited athletes at Cornell than at Princeton. If half of recruited athletes did not submit score, recruited athletes would only represent 7-8% of test optional students at Cornell in the previously referenced test optional years (before test recommended/required).
I was an engineering major and athlete at Stanford. My experience was athlete engineering majors were the exception not the rule. Engineering majors were far less common among athletes than among non-athletes, with the key exception being Management Sciences & Engineering. The larger time commitment and larger number of required classes (including 4-year sequences with little flexibility) for typical engineering majors likely contributes.