If you are going to post something like this why not back it up?
The article you've linked to multiple times in these comments shows a 13% delta between the GTX 1080 and Vega 64 at 4K Ultra (with the Radeon in the lead). The 21% difference is at the High settings Computer Base was testing at.
But where is it 21% better?
It was linked in the article, but here you go.