I own the 800 PF, have used it a lot and do not agree with this observation. It certainly has not been my finding. Nor the finding of pros who have tested it.
I believe you have misquoted Becky.
What she said when shooting it on her Z6 was "either the resolution on here Z6 is not high enough at 150% crop so the images look sharp OR that images of very tiny birds at long distance look soft". Shooting f/8 on a warm day and an undisclosed shutter speed the results will always be an issues.
Kon shooting with the lens wide open on the Z9 at 1/1000 and ISO 400 had no such issues.
So there are a number of potential reasons Becky may have not achieved as sharp results as KON -- AF not on subject, shutter speed too low, heat/haze, Z6 needed to be AF-fine tuned (but this would not address the fact is was in focus at shorter distance).
An album of some of the initial test shots I took near my home can be viewed via this link or in low res below -
Shots taken with Nikon Nikkor Z 800mm f/6.3 PF VR - some with TC14 and TC20.
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12m
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40m
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47m
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12m
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12m
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5.3m