There are no standard conditions for a road-race marathon--that is, you can't point to a set of conditions and say that it is any better or worse in which to hold a race. All competitors are subject to the same conditions.
True that a given athlete may perform best in particular conditions, but there are lots of different athletes.
I personally have an incredible tolerance for heat. I actually prefer super hot and humid to anything else, as long as I have water, and depending on race length. Kawauchi seems the opposite. Conditions aren't relevant to assessing the quality of a marathon race, even if, say, 50% of competitors withdraw. Withdrawal is a choice made by athletes, and is just one choice among many choices that take place throughout a thon, the overall total of which describes the fabric of the race.
It is not equivalent to something like a mudder...nor is it equivalent to a race run on a track. It is somewhere in-between.