The video was a flavored sales pitch that requires reading between the lines and interpreting what is missing out of the interview as much as what is included.

Durability could because of heat build up and operating temperature and have nothing to do with weatherproofing and structural strength. The two examples he gives are places with temp extremes, Antarctica and the Desert.

R3 also has to have a cap on the shoe to be waterproofed?

The 1Dx III has a faster mechanical shutter than the R3.

The real deciding factor for me would be the AF system, which is better?
No doubt the R3 will be better than the R5 and R6, but how does its AF system fair against the 1Dx III?
That is the comparison I want to hear.