It seems to me that painters and composers create art that is appreciated much more widely and over a much more varied audience than software engineers. Given that the inner workings of software are visible to a much smaller audience, isn't it likely that their skills are being over-rated?

I think "10x developer" is a very ambiguous definition, and comparisons in other fields are confused because of this.

I'd rather use the adjective "elite". It's more intuitive and unambiguous.

Carmack is without doubt an elite engineer; his work, in my opinion, is on the same level as Bach's in his field.

Yes, absolutely. If you can create a simple, clean, extensible abstraction that hundreds of other engineers use to avoid recreating and testing functionality this could be considered 10x due to the leverage.