I have to say I completely disagree here. The fact it's rated as IP67 means Apple are claiming it adheres to a known, published industry standard. They are not allowed to make that claim if it doesn't adhere to it.
As for that rating, the 6 is for dust ingress, and is the highest rating, meaning it's completely sealed. The 7 is for liquid ingress and means it is supposed to be able to survive 'up to 1 meter (3.3 feet) of (calm/still) water for up to 30 minutes.' If it fails before reaching those limits, it either a) is a manufacturing defect, or b) shouldn't be claimed to meet those specifications in the first place.
Think about it this way. If you bought a truck that was advertised as having a towing capacity of 5,000 lbs, and you tried towing something that was only 2,500 lbs and it ripped the trailer hitch off, you could rightly claim it doesn't live up to its advertised specifications, and that as such, the manufacturer should fix it at no cost to you. This is because the damage was not caused by you misusing it. You used it within the rated specification. However, their inadequate adherence to that specification means it's on them to address, not you. The manufacturer can't say 'While we've rated it to 5,000 lbs, we will not cover any case where the hitch gets ripped off whatsoever' as then who's to stop them from saying 'It's rated to 10,000 lbs!
Simply put, they are advertising one thing and providing something else.
Specs have to mean something. People make purchasing decisions based on the advertised specifications and with the understanding they should be able to use it within those rated specs and as long as they aren't exceeded, the unit will function as designed. That's why a person purchasing a device rated to support being submerged up to a meter for 30 minutes should rightfully assume rain or splashes, or even a 1-foot dunk for 10 minutes will not be a factor.
Apple got this one wrong. I understand why they're making that claim, but the above should really be quoted to the Apple Store when they are trying to charge you for the repair.