Of course, one can never completely rule out the possibility, but he receives his test samples directly from Nikon, which would be negligent on Nikon's part.
There's a reason a lot of manufacturers supply Christopher Frost and others with lenses for his/their reviews, because he has many viewers and a corresponding reach among potential customers.
Knowing this, it's hard for me to believe Nikon is sending out a bad sample.
In the shorter focal length range, below 400mm, it looks excellent and even at 400mm it's stiil good, it just falls off visibly in comparison to the shorter focal lengths and in direct comparison to the Z 400/4.5, which was at least to be expected.
As good as zoom lenses have become in the meantime, a prime lens in the identical price category, in the case of the Z 400/4.5 even slightly more, will always have advantages in terms of image quality.
You can optimize it for the focal length, where you have to make compromises with a 4x zoom, so the performance will never be identical across all focal lengths.
In return, of course, you get a lot more flexibility and versatility with the Z 100-400.
If it is primarily about the long end, the Z 400/4.5 is certainly the higher-quality solution.