yes, there isn't going to be a significant difference. I'm actually surprised there's ANY difference to a super telephoto, but there it is.It seems obvious they are RF patents based on the 31-33 mm back focus distances specified in the patents.
Based on an appropriately-scaled overlay, the second group back from the front element (G1n) sits ~6mm behind the corresponding group in the 600/4 III, and the next group back is ~12mm behind the corresponding EF group.
View attachment 184781
I think you're expecting a lot from a focal length that is supposed to show no benefits for a shorter registration distance.