That's just more rehashed propaganda from the Apple camp. They said the same thing about removing the 3.5mm headphone jack, until the excuse quietly switched to "it's about more battery space".

Apparently a similar hole into the enclosure for the Lightning port was impervious to water in ways other holes weren't. The RDF kept the water out.

I've owned multiple waterproof phones with a simple o-ring gasket around the battery door. Low-tech I know.