[dcl.init.general] p16.6.1

[Example 2: T x = T(T(T())); calls the T default constructor to initialize x. — end example]

The result object x is not definitely initialized by the default constructor. The result object is actually value-initialized.

If the initializer is (), the object is value-initialized.

In this case, the result object will be zero-initialized if T has a trivial default constructor. This point can be shown by this example.

struct A{
    int a;
};

int main(){
   A a = A(A(A()));  // #1
   A b;
   std::cout<<"a.a: "<< a.a<<std::endl;
   std::cout<< "b.a: "<< b.a<<std::endl;
}

The result printed by Clang is

a.a: 0   // the subobject is zero-initialized
b.a: -1215191136  // indeterminate value

If the default constructor were called for #1, the subobject A::a would be uninitialized.

The comment should just say x is value-initialized.