The earth plane is considered to have zero resistance.....
Which is exactly what I said.
The through-earth electrode to electrode resistance is just the series sum of the two electrode to earth resistances (not counting other parallel electrodes, metallic paths, etc.)
And if the two electrodes are close enough, the through-earth resistance between them will be
measurably LESS than the sum of the two electrode to earth resistances.
Note that the resistance will always be less, regardless of how close or distant they are, but the difference will not be measurable beyond a few zone radii.