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, .