I originally did engage with the comment. Water-resistance absolutely still is physically possible if the replacement battery is waterproof. Water can over time be corrosive at the contacts, but that's a risk for the user. It does not in any way imply that water will enter the internals of the device from the point of contact with the battery. This will require a bit of engineering at the contact to ensure that water doesn't enter the device. As for the size argument, adding 2 mm of thickness is less important than providing five years of extra life.
Wait, are you proposing sealing the phone and sealing the battery separately, but not sealing the contacts between them? That’s… super sketchy for salt water immersion. Unless you add fuses and a BMS and safety mechanisms into the “battery”. In which case wouldn’t customers want to be able to replace the actual battery within the now-a-battery-plus-computer phone accessory once it wears down?