The way I see this he is only talking about a circuit that has it's overcurrent device in the second panel. Electrically that is a switch loop. There will not be any inductive heating problems with that arrangement.
I assume the neutral and grounds are separated in that second panel but he is still not imposing any objectionable current on the EGC so that concern is allayed.
250.4 refers to the EGC and you have a lower impedance to ground in the main than you do in a sub.
By wire nutting an extension on the neutral you are simply adding 2 more possible places for this to fail.
The bottom line is, if the AHJ wants it you probably have to do it but I am not sure what the justification would be beyond that.
The only case I could think of would be if this is a multiwire circuit (240.4(D)) but even then the code would make something less safe by adding one more connection than was necessary.
If this is a pure 240v load there wouldn't be a neutral anyway. Maybe the trick would have been to swing over the water heater.
