Why is my abdomen much cooler than the rest of my body after a ride?
This part of the body isn't doing much, unlike other areas, so isn't warmed by blood flow and muscle activity. It is exposed to the wind, and can get a little sweaty quite easily, even if the ambient temperature isn't all that high, but it's also sheltered from the sun. The climbs probably got you sweating, then the descents would have significant wind chill. 

Even if you don't have fat to insulate your digestive system, there's still a layer of muscle between it and the skin, so the skin temperature there isn't the core temperature (if it was, there wouldn't be so much need for internal medical thermometers). 

Don't listen to whoever told you warm water doesn't get absorbed. It's hard to prove a negative, but if you've ever ridden all day in the warm with only warm water available and no need to go to the toilet despite drinking loads it will be obvious that it's getting absorbed (even if it does turn straight into sweat).