I agree with you. Dave seems to view culture as a completely blank slate. In his view, things are either clearly non-cultural (not being able to run a marathon due to leglessness [which btw is only an issue in cultures where marathons are a thing]) or entirely cultural, which he implicitly equates with “arbitrary”. If it is cultural then someone willfully instituted it. His style is representative of the intellectual culture he draws from. Steve Pinker wrote a great book about this called “The Blank Slate”.
I see culture a tree of branching possibilities that grows on biology (among other things). Not every branch is equally likely. Some areas of the humanities insist that there either is no tree (culture is entirely constructed, i.e., arbitrary), or that - even if there were a tree - we could never even begin to understand what it was like.
