I've only ice fished a few times & that was many, many years ago. We don't get that much ice here. I used to work for a road construction contractor & we would push snow in the winter. Some of us carried pints of flavored Rum or Brandy to ward off the cold. I liked Black Berry or Peach Brandy. It didn't really make us warmer, we just thought it did. A pint would usually last several days, as it was used sparingly.
I can see limiting even the alcohol in such competitions, for no other reason than safety, and can understand the premise of anti-doping, but cannot understand why anyone would think it can give them an advantage for fishing.
However, when there's money involved, some will do anything I guess!