People are going to be disappointed if the Z390 turns out to be just a rebadged Z370...Also you are correct on all counts.
Apparently Z390 is a rebadged Z370.... so effectively a rebadged Z170....
https://wccftech.com/intel-z370-rebadge-z390-8th-gen-compatible/
And this because the original Z390 was a 10nm chip that wont come out.