The cooler has nothing to do with dust particles getting inside. I merely blows air on the phones surface, not into the phone.
My guess is that there's an issue with the two specific devices mentioned in the xda thread. It's up to the ASUS service technician that investigates the device to determine if it's warranty or if the user has exposed the device in conditions that are not covered by warranty.
There's nothing strange about moisture getting inside, they way he described it. A rain jacket pocket, If it's is not a very premium jacket that breathes, will keep moisture both on the inside (from your body) and on the outside. This is in a way, worse than being hit with a splash of water if being exposed to this environment for a longer time. But as he reported, it normally goes away after a couple of days unless circuits have been permanently corroded or short circuited.
But to answer your question, if your phone has only been in your pocket and have this issue, then yes, it's covered by warranty.
