Read it carefully, it says when using two ungrounded conductors and the neutral of a wye system.
In that situation because of the 120 degree phase angle the neutral doesn't carry imbalance current of the phase conductors like it does with a 180 degree phase angle. If you have 10 amp line to neutral load on A and 10 amp line to neutral load on B the neutral will carry approximately 10 amps as well, but with same connected load in a single phase application with 180 degree phase angle that neutral is not carrying any current.