You are right: If this trace would be a part of the ground plane, then enormous voltage drops due to very spiky rectifier currents could contaminate signal ground. Remember, current spikes of several Ampères can flow when the rectifier opens for a short period.

Differently to high frequency ground return currents, which gather nearly directly under the signal trace in the solid ground plane, and this the more the higher frequent the current components are, at rather low frequencies (remember the current spike lasts a few milliseconds) a different behaviour can be seen: The whole ground plane is involved by the current flow. So, even at a distance of several centimeters a relevant current will flow and if this current is rather high, then finite resistance of ground plane will be responsible for considerable hum harmonics superimposed to ground.

Where very high currents are flowing on a board it's wise to split the ground plane, so that the high currents and sensible parts of the circuit don't share the same ground.

Kai