It's not correct, I'm certain as well, I just put there simple multiplication for anyone to see and do.. There are charging efficiency losses, and a 400 w / hr constant electronics overhead, and 3 and Y are not the same, and no exact version of S nor X is exactly 3 mi/hr, and so on. So the ballpark was 19.2 for Model 3, vs the table saying 18.5, I'm surprised it was even that close. Close enough for government work, as they say...I'm certain that's not correct.
This is the Model 3 section of the forum, so let's do that one first:
Splitting between the 15 mph and the 22 mph would be about 18.5 mph.
The Model X is a bigger and heavier and less efficient vehicle, so with the same amount of energy will get fewer miles, so it will have to be less than that, not 19.2.
Splitting between 8 mph and 14 mph is 11 mph.
And this is why it's so frustrating that Tesla did this idiotic mistake of releasing the Gen3 wall connector when shared charging was not implemented yet. People are scrounging for used Gen2 wall connectors just to get this basic useful functionality.
I guess I'm not really sure why the OP dismissed the suggestion of the Tesla Gen2 HPWC's. From what I understand if configured at 40A, it would do exactly what the ClipperCreek does, split it 20A/20A when both cars are calling, and then when one car is done allocate all 40A the the remaining car. It's a "dual charger" that comes in two separate identical boxes. There is even a metal outside pedestal Tesla sells that lets you mount them back-to-back, makes the entire thing like one physical device.