On the other hand, I can certainly imagine situations where the requirements aren't "hardcoded" but rather they are flexible and heavily driven by what's feasib

On the other hand, I can certainly imagine situations where the requirements aren't "hardcoded" but rather they are flexible and heavily driven by what's feasible and what's hard from the engineering perspective (so they can't be "pulled out" from the product leader), and determining that vision properly requires involvement of engineers and can't [technically can, but the outcomes suck] be prepared by someone else and just handed off to engineering.