Has the freezing point of water changed at some point since... whenever? No. Then how the hell was New York only 4 or 5 degrees colder than it is right now yet covered in 2000 feet of ice? (Its avg worldwide temps duh but the graph doesnt really specify that when talking about NY)
Furthermore, no one actually measured air temps back then. You can GUESS based on lots of things, but some of the same things those guesses are based on are also guesses themselves.