[expr.prim.id.general] p2 says:

If an id-expression E denotes a member M of an anonymous union ([class.union.anon]) U:

• If U is a non-static data member, E refers to M as a member of the lookup context of the terminal name of E (after any transformation to a class member access expression ([class.mfct.non.static])).

[Example 1: o.x is interpreted as o.u.x, where u names the anonymous union member. — end example]

IIUC, the intent of the first bullet actually means the id-expression that would denote the member of U(conceptually designated by u.x) will be reinterpreted as o.x as if x is the member of o.

Consider this case:

struct A{
   union {
    int a;
  };
  void show(){
    int c =  this->a;  // #1
  }
};

The id-expression a denotes the member a of the anonymous union, and the terminal name of a is a, whose lookup context is just A, as per [basic.lookup.qual.general] p2. Hence, the intent of the rule may want to say a is interpreted as the member of A.

However, the note conveys a converse meaning with the intent, I think.