Never said it was better overall.
The mixture is an azeotrope so you are correct, boiling point would be higher. Although, with the system under pressure the BP of pure water would be raised quite significantly.
Pure water is still a much better cooling agent than the ethylene glycol/water mixture though. Running an engine on pure water temporarily will have no harmful affect on the engine relative to temperature. It would actually run a little cooler.
At work we run pure deionized water through everything; copper, stainless steel, steel, aluminum, plastic etc. When the parts are serviced, they look brand new. Can you believe it? The difference is that our system has resin beds that constantly remove ions from the water. It's not the water that causes corrosion it's the ions that are dissolved in the water. The required heat transfer from equipment to coolant is very high so a glycol/water mixture would be detrimental. Also, the equipment never sees freezing temperatures making pure deionized water the perfect coolant.