How do you know the Sony at 600mm (f/6.3) is as sharp as the Canon 100-400mm at 400mm (f/5.6)?
There is no published comparison side by side. As I wrote above, the MTFs published by Sony are not real in practice ones but arbitrary theoretical curves that do not take into account diffraction or real life features and are just based on Sony software and are not validated by CIPA or anyone. That is not evidence to compare two lenses from different manufacturers products. Also direct observation from the Optyczne review is that there is a real observable increase in sharpness on stopping down.
As for all lenses improving on stopping down, I have had 3 copies of the 100-400mm II and all have been sharpest wide open, and decent testing sites find no significant improvements in their MTF tests, and they are about the same at f5.6 and f/8.
https://www.opticallimits.com/canon_eos_ff/896-canon100400f4556is2?start=1
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You are accusing members of CR posting myths from the internet, but where is your real evidence?