Bottles is a noun and not an adjective.

Also bottles has no IT related meaning, while immutable does.

“Immutable OS” is not a product name.

An i"mmutable" OS becomes mutable whenever a user wants to change anything on it.

Now imagine I keep describing my car as undrivable, because it only becomes drivable when somebody gets in and drives it. - You’d think that this is a completely deranged statement.