IP68 means it can either let water in and have no harmful effect or not let water in, that is up to Samsung to define. It however does not allow Samsung to say it has an IP68 rating, if testing under those conditions show the device to fail.
The WHOLE point of the rating system is so that companies can't falsely advertise with a catch all "water-proof". A specific rating saying exactly what the tested device can go through.
The real defense for samsung in these cases is that they can say previous drops impact a devices ability to withstand water intrustion, thus makine IP rating only valid for "virgin" devices essentially.
Samsung is getting out of a lot of the water warranty claims by not performing non-warranty fixes. So when people have to replace a broken screen, they go 3rd party, automatically voiding the warranty. Rather ingenious of them.