They could have been, but they absolutely weren’t.

I had a phone with a easily replaceable battery and it was waterproof. You say these weren’t common though they were. It’s often said that not being able to replace the battery is needed for helping things to be waterproof but that’s really not my experience, nor that of many others.

This claim comes up pretty much every single time. On a Dutch site people often give detailed answers on which phones they had that were waterproof and so on.

more water/dust proof phone

Phones were just as waterproof and dustproof when the battery was replaceable.