if static classes can have only static members, how about nested types?

a static class can have only static members. but it also can have tested types which would not be considered static.

proof that a class nested inside a static class is not static is that it can have non-static fields.

so a class (or any other type) nested inside a static class would be considered an instance member of the static class?

A nested type, by itself, isn't a member of the parent class. That is, it declares what the nested class is, but it doesn't create any instance of that class.

If you were to make a member variable that could hold an instance of the nested class, that variable would need to be static, because it's in a static class.