Being "good at engineering" doesn't change the laws of physics.

Those Nokia phones were not waterproof. Also, I'm pretty sure they were thicker.

An o-ring only works if the battery cover is rigid enough that it will not flex at all even if, for example, you drop the phone in cold water rapidly cooling the battery cover while the internals stay warm for a minute or two.

The battery cover will change size slightly with the temperature change and no screw can be strong enough to stop that. Covering the entire battery cover in glue and attaching it to the battery though.. that will eliminate the movement.

Perhaps Apple can find a water proof battery. But there's no way they can keep water out of the battery compartment while being user serviceable.