As far as gelding having to wait until it's warmer, I had a vet tell me that once about an elective surgery on my horse. His point was that he needed to do the procedure with the horse on the ground and the cold ground made it more risky on the horse. Secondly, we needed to flush the wound with generous amounts of running water from a hose.....definitely not something you want to have to do in the winter time. This surgery was a trephination (skull surgery) to remove an impacted molar. A gelding procedure, especially in a mature stallion, necessitates lots of walking. My guess is that the vet is thinking that he'd like to stack the odds in favor of three things that are important to the success of the surgery -- good operative view for careful surgery and control of bleeding on a fully developed male, hosing for a clean post-op site, and mobility afterwards to keep the swelling down.
Of course, if the owner has great facilities with a warm barn for the procedure, a well equipped washrack with cool - not icy - water, and were a good horsewoman, it would make a difference. But we already know she's not a good horsewoman since the vet hasn't seen this guy since he was a baby.