They could probably fix it in software by taking snapshots of the page every 17ms and only displaying those static snapshots. That, of course, would lead to jerky scrolling instead of smooth scrolling so people wouldn't like that either. That would work best on 120hz displays.
Apple tried to make the landscape scrolling perfect and screwed up the portrait scrolling in the process. If they had continued to make portrait scrolling perfect as before, landscape scrolling would be defective but still much less annoying to those who notice these things. Engineering is all about making choices between least bad options like that, they chose poorly based on use cases.
I'm still waiting for mine to arrive. I expect I won't notice as my vision is declining due to age and my field of vision likely won't see the complete line all at once like I used to be able to do. I have to scan while I read even on narrow displays. Also I plan on mostly using landscape mode so I can expand the text sizes more to see better. Still won't know until I have one to try. I am expecting an improvement over my 13ProMax which does work fairly well for me now in landscape.
Because the mini 6 is taller than the mini 5 the jelly scroll would have been even less noticeable than the mini 5 if they kept the landscape orientation.
Instead they changed to make the jelly visible in portrait AND also made the mini 6 the narrowest iPad ever so that the jelly effect is the greatest of any iPad in their entire history.
Absolutely stupid decision. Makes no sense whatsoever. So it must have been cheaper.