Yeah, the Celsius is fishy. It really is Kelvin plus a fixed number (the difference in K between the absolute zero and the tempature of freezing water). So, it should be something like K[Celsius] or so. But it isn't. Now we are stuck with either bad notation.
What's the consequence of leaving it the way it is and you have to map over? The consequence of changing it for the Standardized Vocabs is probably not that much: We'd have to deprecate the existing one, create a new one and link between the old and the new. Not sure how it would work on existing ETL code, and even how many people have Celsiuis to begin with. You probably get that from measuring body temperature. For US databases, you mostly get Fahrenheit back. In future, when we start fixing the unit mess, we will have to map and convert to Celsius anyway.