But there is at least some argument that smartphones nowadays have some pretty crazy waterproofness that I'm not sure is physically possible with a replaceable battery?
In more concrete terms, there are modern (2025) smartphones like the Samsung Galaxy XCover7 Pro w/ replaceable batteries which have IP67 (1 meter of water for 30 minutes) or higher certification. The back panel popping off too easily (and dumping the battery with it) is one of the common cons with the device though, so while you can get everything it is an extra layer of difficulty to try to design around correctly.