Have you ben drinking this time? If so i want some
Those slots allow adjustability so you can position the rack as needed. Those fasteners, the big and small, are not in sheer unless you bottom out the adjustability… you are keeping them in tension and clamping those faces together creating the friction.
I have some in the shed i can pull out. But if i remember correctly that shape doesn’t induce any extra moment in the bolt holes like the prinsu style mounts do. Look at it from the side and draw a free body…