Of course they will need to improve the AF - what 'as good as' means, I don't know. I think most of the AF improvements in the Z9 come form the Expeed 7 and the additional SW it enables, not so much the stacked sensor, which is more about the continuous VF feed. But Sony and Canon have made the AF close to the level of the top end, so I don't see why Nikon shouldn't.
It has nothing whatsoever to do with a 'mirrorless D850', it has to do with Nikon improving the AF across the range. Nikon shouldn't behaving a third go at a camera that is over five years old now, they should be looking forward and producing something different. The Z9 covers both the D850 and D6 slots in the mirrorless range - it can do anything that either of those could do. There isn't space for a camera that does a bit less - rather they need one that does something different, just like the D850 didn't do the same as the D5 or D6.