I think what he is trying to say is this. When someone says it’s 0 degrees Celsius, that also means it’s 32 degrees Fahrenheit. However, if you were to convert 33 degrees Fahrenheit to Celsius, it would be 0.556 degrees Celsius. So if somebody asked you for the temperature, you can’t say it’s 1 degree Celsius, because that would be wrong. It would also feel odd saying that it’s 0.556 degrees, so you would feel inclined to say that it is 0 degrees Celsius, which is technically incorrect. (If I am wrong, and it turns out that people actually DO use decimals when saying the temperature, please let me know).

I don’t know whether this cleared things up or if it confused you even more.