It doesn't complicate moisture seals. A gasket is very simple. You can double gaskets as shock-reduction, too, which is impossible with adhesives. There are already about 30 screws in an iPhone 11, I don't think 4 more will make too much a difference, but if you were really trying to save space having two components use the same screw is possible.

This really isn't a total redesign, by the way, and it doesn't just apply to the battery, but helps repairing everything else. In any case, it absolutely isn't a total redesign, and FWIW the iPhones don't even have that much battery compared to more repairable phones of the same size.