That isn’t a be all and end all test, it is just a quick indicator.. kind of like the pop-up thermometer in a turkey.. it helps you determine if it is properly cooked, but it doesn’t guarantee it. I can’t tell you how many phones I’ve lost due to water damage.. I know enough to pop them open and see, most folks I wouldn’t recommend that.
I agree it is the low tech version of the idiot light and only a rough metric (I have an undergrad in EE), but I expect a company to stand by the word of it’s agents and if they are willing to make a diagnosis and promise based on a poor indicator (and I told them it may have gotten wet) then they should eat the costs when they are wrong.