Well, tap water does have non-water things in there, but it generally doesn't matter that much. If it was flavoured water though, you may have issues with sugar.
I can't think of any other relevant kinds of corrosion relevant to a keyboard, with just water as a factor. Maybe some other type could happen if instead of water I poured in sulfuric acid or something. You are generally correct that rust, as a technical term, relates only to iron, but people use it anyway with other stuff.
Keep in mind the layout of space invaders, I'm not sure what I did would even get into the switches to begin with, and even if it did, the only thing that would be hit by the water is possibly the spring, and the very bottom of the contacts - not even the part that actually makes contact. I know from the color that the contacts are not iron, and it would be really weird if the springs were iron and not some alloy. Alloys are generally resistant, and running hot water to remove all coffee related chemicals by dissolving them for not even a minute would never cause lasting damage, nor cause it to magically begin to degrade at a faster rate than the gas in the atmosphere where it is already was.
That said, cherry switches could be less waterproof. Although I'd be more inclined to believe that the modern controller in his case got hit. Besides, it's broken now, it can't get worse than broken.
Check links for direct examples/testing, rather than what I'm asserting based on my knowledge of chemistry.
http://imgur.com/a/9sHx7http://imgur.com/a/cExIcKeep in mind that the rust testing is with continual and complete exposure to the elements, which includes things like fog, misting, and various dry/wet phases, along with whatever substances that the water from the weather would contain.
