That overboard discharge sea cock would only be open for a few minutes when you want to pump the tank overboard beyond 3 miles. The rest of the time it would normally be closed.

Having the vented loop means the pump's check valve needs to resist the pull of a deck pump-out even when the through-hull is closed. Without it, the closed sea cock will provide even stronger backup resistance behind the pump to block the suction from the pump-out boat. The vented loop only serves to prevent siphoning into the bladder tank when the overboard discharge thru-hull is open, and that is accomplished anyway by the pump's built-in check valve. If the pump check valve fails you don't need to worry about siphoning/sinking because you wouldn't leave the overboard discharge open anyway, unattended.