It's not about warranty, it's about it not breaking.
IP68 says that the device can be immersed in 1.5 meters of still water for 30 minutes and not sustain any damage. That is what the Galaxy does, and that is what the Galaxy has been tested to do.
Salt water causes problems, as does moving the phone around in the water. There's a big difference between dropping a phone into 1.5 meters of distilled water, and running into the surf (into salt water with waves) with your phone. This was what Samsung depicted in their advertisements, this is what Samsung did not test, and regardless of if Samsung still repaired their phones under warranty after doing - this is what Samsung Galaxy Phones were not tested for.
Australia has some pretty oppressive consumer protection laws (oppressive on the seller, great for the consumer). In that "misleading and deceptive" doesn't actually have to mislead anyone, nor does anyone have to actually be deceived by it. It only has to be proven that it a reasonable person could have been mislead. This is an important legal distinction, because the prosecution doesn't actually need to prove damages - they only have to prove that people reasonably could have been mislead.
Samsung is likely going to defend the case by saying they still honored warranties, and they didn't punish customers for saltwater damage.
ACCC is likely to argue that's irrelevant, because it was only tested to 1.5 meters in still fresh water, and they have examples of where Samsung customers used their phones as depicted in their advertisement and the handset failed (and needed to be repaired at Samsung's cost).
Lastly, given that the ACCC has a reputation of being a toothless tiger in Australia, you can bet the reason this is going to court has absolutely NOTHING to do with consumers complaining - and everything to do with Samsung being dobbed in, then the ACCC being pressured by a large multinational organisation that sells phones that competes with the Samsung Galaxy (which rhymes with either papple or goodle) , and is currently advertising their handset in accordance to Australian Law.