Yea, wondering the same. We do not have enough data to come to this conclusion, if anything in my opinion the oposite is more true.
The only reason to think big Vega will be 1080 performance or less is you just look back at Polaris. Before that AMD have always managed to be close or beat Nvidia. Vega is very late so it should easily bring 1080 performance, but I expect 1080Ti performance at least on AMD games from big Vega.
Vega is a big tech arc upgrade for amd.
drawing conclusions based on amd sandbagging the industry again is foolish.