Comparison is totally valid
I'm not suggesting that the Zenbook is a bad machine--quite the contrary. If you want an ultrathin laptop and need/want Windows, I would recommend the Zenbook.
And if the Zenbook's similarities with the Air were limited to the overal design and wedge-shaped case, I wouldn't have called it a copy. But given the nearly identical motherboard, IO board, SSD placement, battery, speakers, cooling assembly, and aluminum unibody case, I believe the words "copy" and "clone" are appropriate.