That's only the case for a public transport flight. While I would be the first to suggest that the ANO would be better for requiring a commander to be designated for all flights, it offers only:
Pre flight action of commander of aircraft appliys to all flying machines, it would be difficult to interpret otherwise. Certain actions must be carried out before flight unless you have access to air to air fuellling.
Oh and of course 'booking out'
reading that back i havnt made that clear.
If the law requires a commander to take certain actions before flight it is common sense to interpret there must be a designated commander before these actions can be taken!