Celsius and Fahrenheit are both degrees units, meaning they’re scaled off of some arbitrary temperature. 20°C means 20 Celsius units away from Celsius 0.
Kelvin isn’t scaled. 0 Kelvin means there’s no temperature energy at all. So there’s no need to couch it in degrees, since it isn’t arbitrary but a defined scale with an absolute zero point.
I like to think that he forgets, keeps trying and then makes a new post about it