this pointer cannot be aliased in a constructor:

I am learning about inheritance in C++. And i came across the following statement:

In other words, the this pointer cannot be aliased in a constructor:

extern struct D d;
struct D
{
    D(int a) : a(a), b(d.a) {} // b(a) or b(this->a) would be correct
    int a, b;
};
D d = D(1);   // because b(d.a) did not obtain a through this, d.b is now unspecified

The above example is from cppreference.

My first question is that it is written that "this cannot be aliased in a ctor" but then in the example above, they've written in the comment "b(this->a) would be correct". This seems to be a contradiction to me because when they said that this cannot be aliased in a ctor i understood that "this cannot be used in a ctor". But then why are they saying that writing b(this->a) would be correct if this cannot be used/aliased in a ctor. Isn't the initializer list considered "in a ctor"?

---

Now lets look at a custom example:

struct Name 
{
    private: 
        int x = 0;
  Name(int n )
  {
     this->x = 4;  //is the use of "this" well-defined here?
  }
};

My second question is that is the use of the expression this->x inside the converting constructor shown above well-defined? I mean since according the quote at the beginning of my question this can't be aliased in a ctor, so this shouldn't be valid.