He undermines his own argument here:

"And it could even be that Tut did not have the eye shape himself, if his mother was a woman other than Nefertiti who did not have it or if the trait was passed only from mothers to daughters. In either case, Tut could be shown with it simply as an artistic continuance of the characteristic."

IOW, if it's there, it's either real or artistic license. And if it's not there, it's either not real or artistic license.

So, the only evidence is that it appears on many images of Nefertiti. But the same effect appears on somewhat earlier coffins, ie the Thuya coffin. It might be possible to say that the artistic *convention* of showing the slanted eyes came about either late in Amenhotep III's or early in Akhenaten's reigns, but even then that might be reaching.

Because if they are going to say that the slanted eyes are representative of Nefertiti's actual appearance, then they will also have to say that the extreme heads & bodies of Amarna art are also representative of the sitter's appearance.

Stylistically the KV 63 coffins fit nicely into the end of the 18th D. Anything beyond that is pure conjecture.