Not a joke, the AU is a pretty common unit of measurement when talking about distances too large for metres and too small for light-years, especially since it's based on one of the most widely understood astronomical distances. Of course using it to describe the distance it's derived from is more than a little tautological, but that wasn't too important in the context of discussing magnitude prefixes.

It's roughly the same thing as describing a distant potentially-habitable planet as being "1.5 times the size of Earth" instead of giving its radius in metres. It's just that AUs wrap that "n times the distance between the sun and the earth" into a nice standardised unit.

...thus making it tautological and extremely pertinent given the context