Keep in mind that when the watch is sitting, it's not in active use. Now I have a docking cradle on the dash in my car for my iPhone. Depending on the position of the sun, there are times of the day when I can't drive with it on the dash because of the sun. It doesn't take long at all to overheat such that I can't use it, and it takes a lot longer than just a few seconds of being in the shade to resume working. That tells me that any realistic exposure to heat while wearing the watch is going to take longer to dissipate than a few moments of protection the shadow of your body happens to provide, especially if the ambient temperature is quite warm as well. Not to mention, when being worn, the watch is actively being used.
You also failed to mention Apples warning to avoid rapid temperature and humidity changes. The kind commonly found in the Summer along the Atlantic seaboard, where outside temperature and humidity can be quite high, and can instantly change to very low temperature and humidity inside an air-conditioned building, and vice-versa. Add to this something like a dry 90 degree day at the beach on the Pacific coast, and submerging the watch in 50 or 60 degree sea water.
So we can speculate all day whether the watch will survive any of this, but until we have tests, all we have is Apples warnings, and behavior of similar electronics in similar conditions.
It's clear to me from all of Apple's disclaimers, that the Watch may be engineered to a higher standard, but it's really designed for the office environment, or running on the treadmill at an air conditioned gym.