I know that the sheathing is designed for exterior use because the manufacturer says so. I also know from personal experience. It is not "treated" and I never said it was.
It is ,however, very water resistant and can withstand multiple soakings with little loss of structural value. Yes, treated wood is used where it will be in continuous contact with moisture. That does not mean that untreated wood is not extremely water resisistant. It is.
It is primarily the water resistant glue that is used to make exterior sheathing that makes it so durable. Interior stuff is use for cabinets, furniture, and the like and uses ordinary glue. It it totally different stuff which would never be used as structural sheathing.
So, when you buy some plywood called CDX you know it's exterior rated because of the X. Actually, all sheet stock used for sheathing is exterior rated. That way, the carpenter knows that he can go ahead and perform the task without worry. It is assumed that if the sheathing gets wet it will eventually have a chance to dry out. Then it is fine.
I have been on many sites where it rained and rained on half-finished houses. Yes, it is better to work with dry wood but the wood is not ruined, not by any stretch of the imagination, neither sheet stock or solid wood. I have seen shacks build with X rated plwood that have lasted for years without any further protection. Bottom line: the stuff is durable because it has to be.
In this case, we have one wall of the house so the sheathing is further protected by the entire house and the overhang above it. Only a hurricane would have any chance of significantly damange this exterior rated sheathing. In the very unlikely case that it were damaged it can be easily repaired. Sheathing is commonly available and simple to install.
Actually, it is far more likely that the existing sheathing is damaged already. It is quite common for this to happen since once water gets in it can have little way to get out. I say tear off the siding on the first dry day so than an inspection can be made and repairs planned. I do not find it necessary to work in the rain but it has nothing to due with the material getting wet. It's just because I don' like to get wet, :).