Yea I'm not sure what would be crazier on it's own there, the late 100+ heat or seeing the snow. Obviously the combination of the two things within a day is insane and historic, especially in terms of temp differentials and departures. But honestly the heat wave part of this is likely the more extreme thing. It's relatively normal for the Rockies to start seeing higher elevation snows in September and Denver in fact averages 1.3" in September. It would appear that the only snow record Denver would break looking at what's being forecast there is possibly the heaviest first snowfall of the season. The earliest is 4.2" Sept 3, 1961.
https://denver.cbslocal.com/2020/09/04/weather-whiplash-colorado-record-heat-cold-snow/
This article also mentioned the last measurable September snow at Denver occurred September 24th, 2000. So this hasn't happened at all in September in the last two decades.