I can't quote properly box of boxes.
So excuse me in advance if i miss something.
1, radar cross section depends on wavelength.
L band isn't a silver bullet(it doesn't allow engagement on its own), but "marble" return won't work anymore.
When not on its own - it works. For example, when engagement is done by a missile with an active seaker(if seaker is going to search in the right place). Or when L band track is used to que X band engagement radar on target, to search in a narrow sector.
2, numbers for S-400 radar are appliable only for generic airspace observation mode. And they are for illustrative purposes only(if anything, because f-35 and f-22 ratio doesn't change at all between two pics)
Engagement can be performed much further if target is tracked by other radar(including L), because, again, narrow sector scan.
3, higher sensitivity(50/80-90) matters when there are false objects to see, i.e. there is a significant signal interference. Original talk, as you've just quoted, was not about whenever s-400 can or not engage f-35, it was about necessity of ew support for f-35 force.
4, 50km range woupd be a signal what heavy AA system is almost pointless waste of money, for it can't even stop glide bombs from being used.
Yet there is no apparent push to scrap s-300/400 family of systems.