> If someone was to ask me how far is was from sun to earth, and I answered 1AU, and they asked what an AU was?

You'd probably do the same thing that you would do if they asked what a km was: keep changing the units until you get one that they recognise, and then now they can understand the original unit you used.

(Although in this specific case given that they're asking about the definition of the unit you'd probably provide that context without needing further prompting in the first place, similar to if they asked about the circumference of the earth and you gave an answer in metres)

pretty sure it was a joke

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