the manual is not so clear. Looking at the above pictures, my opinion is removing the 2” couplings and the drain plug is sufficient for emergencies like in Texas a week or so ago.
One reason the black drain plug might not be sufficient is in an emergency loss of power and air temp below freezing, the small orifice might be clogged with a thin layer of ice and keep the manifold from draining. I’ve seen this happen on pool pumps. When this happens, I have placed a small screwdriver in hot water the gently touch the opening. It normally immediately melts the ice and draining occurs. With an expensive heater, removing the 2” pipes allows air to enter and drain quickly.
this is only my opinion

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