Much of the farm equipment you refer to that have 6.7 & 8.3 do not have an exhaust brake function.
So yes, they would not recommend using an exhaust brake for warm up. Additionally, most farm equipment would not see freezing starts
No farm or construction equipment has an EB that I've ever seen. Not exactly necessary when your top speed is generally under 10mph

The point is they specifically advise against cold engine idling. And if you don't think farm equipment doesn't see cold weather starts, come on up for a week in January. I'll dispell that myth for ya. Farmers dont have the luxury of not farming just because the weather turns cold.