GND plane and reference potential
It's all dependent on how the ground plane is constructed, any material has resistance, which means any current flowing in a ground plane will create voltage. For most designs this can be approximated to zero volts. For example 1oz copper has 5mohms of resistance per square inch. 10mA of current would produce at most 5uV of voltage, for most designs this could be considered zero. this means the ground plane can be represented by a single node at DC. High-speed signals need a transmission line to propagate. However for a microstrip line we simply need a reference plane not a ground plane, in the example above it doesn't matter what plane we use, as long as there plane is capacitively coupled to the source a load of the transmission line.
In fact at high speeds just a copper plane cannot carry high frequency signals because it has too much inductance.
For high speed signals the current follows the path of lowest impedance so to create low impedance it is necessary to create the right amount of capacitance between the microstrip line and the ground plane. When the micro strip line is sized correctly with the right amount of capacitance per unit length versus the right amount of inductance per unit length the wave can continue with very little loss. Most of the energy of the wave is not carried by the electrons but in the magnetic and electric fields created by source and carried by the transmission line.