A couple of thoughts here....
I consider a neutral to be a current-carrying conductor. Multi-wire branch circuits only are a means to reduce the number of current-carrying wires.
Generally, running a smaller neutral is a bad idea, and best avoided, even where the code allows. I will concede one such application, though.
Many industrial machines have nothing but three-phase loads, except for the controls and, maybe, a light for the operator. I have seen such equipment set-up with the control/ light power being taken from one leg, fused on the machine, then only a very small #12 neutral run back to the panel. It does look a little sily, that #12 coming out a pipe with a set of #2's- but was once a fairly common arrangement. (This arrangement is disappearing, a victim of the computer age).
Now I think I see where your issue originates. The code wants you to begin de-rating at four conductors....which seem kind of silly when the "power" wires are #2, and there is only that little #12 neutral. I think this might be where that exception is meant to be applied, and no de-rating is needed.
For a conventional branch circuit, there is no doubt- the neutrals carry current.
Consider the following example...two circuits, different phases, one using 10 amps, and the other 9.
As a multi-wire circuit, the three wires carry 10, 9, and 1 amp respectively.
As circuits with separate neutrals, the hots carry 10 and 9, while the neutrals also carry 10 and 9. There's a lot more amps running through that pipe- so de-rating makes sense.
