Also, this entire argument feels like it's getting way too much significance. It's definitely doable, even with IP68, there are tons of replaceable-battery-operated waterproof devices out there, this shouldn't be a reason.
Buyers would also rather keep the solid slab with no hatch form factor, than see the phone be 2mm thicker I assume. A lot of other devices don't have a size budget that is as extreme as a modern smartphone.
Put the other way: a slightly bigger phone (because of some separating compartment, gaskets, battery in a plastic cartridge and so on) could instead have been a solid slab with a slightly larger battery, so several hours extra battery time or higher performance at the same battery time.
I don't trust water resistance well enough to intentionally get the phone wet. But I do think it's probably good enough to greatly reduce the odds of damage in case of an accident.
BTW, I do mean in the pool, as in submerged.
That, and also that they were denying it, yes.
EDIT: And since the battery is non-replaceable, basically that also means with age of phone it just gets slowed done. Did/Does Apple kill the throttling if the phone is on the charger?
Far from it - as demonstrated by the fact that Apple ran a $29 battery replacement program for this very issue.
And $29 is still four to five times as much than just buying a battery and doing it yourself.
Point is: It costs Apple more than $29 to replace a battery and not make a loss while doing it.
So ok, a battery won't cost $6, but a technician's replacement usually won't cost $29, either.
That means, the true costs are more likely to be $20 vs. $50. (And those $50 are brutally cheap, what are they paying their technicians?)
EDIT: FWIW, a "sketchy" replacement battery for the Samsung Galaxy S5 costs between 7-15€ on the German eBay. That was my reference, I don't use Apple products.