I suppose it is literally true that the X-29 is "an impossible fighter jet", in that it was never put into production and fitted with weaponry.

The phrase "inverted wing" is more troubling, because of the potential for confusion with actual wing design terminology: https://en.wikipedia.org/wiki/Gull_wing#Inverted_gull_wing

Perhaps this link could be migrated to style.ycombinator.com?