Polarity does not change, but the direction of current through the coil does change by changing its origin.
In a one dimensional world the only way to change direction is to reverse direction, but thats not true in a multi-dimensional world.
The change in current direction through the coil-electrode is accomplished by switching which end of the wire is connected to the anode or cathode, but current flow between electrodes across the fluid medium does not reverse directions. Anode and Cathode do not switch, but direction does change. Current flow through the wire changes direction without changing polarity, and curent flow through the fluid medium does not reverse direction.
A wire may be a single dimensional current path, but an electrode is not, and neither is a fluid medium capable of carrying current such as water with an electrolyte added. Think of the path as everything between origin, and destination. The path of the flow changes, but the origin and destination do not switch. It goes from the same place to the same place, but leaves and arrives by different paths. In a solid circuit the path never changes.
You can't change origin and destination without switching polarity with either DC or AC. The four wires from the SDC power supply allow changing both origin and destination through a fluid medium capable of conducting current without switching polarity. Its not AC because anode, and Cathode never switch, and its not DC because origin and destination do change.
Its all about controling and using the additional dimensions of a fluid medium. While the nature of fluid conduction is understood controling it through switching without reversing polarity is new and unique.
The point is that, the way the four terminals are switched, the current WITHIN ONE ELECTRODE reverses direction, and has zero average value at the center. At the same time, the voltage BETWEEN ELECTRODES is a constant DC value (ignoring switching transients). Thus “multidirectional DC” refers simultaneously to the multidirectional intra-electrode current and the single direction inter-electrode current.