That isn't wild theorizing. It is solid constitutional law.
For instance, the Constitution provides no right of procreation. Most of us would concede it a right of people. So did the Court when the question arose.
The Consitution does prohibit government from infringing on some of our rights, and it gives Congress some powers to protect others, but it grants no rights by itself.
Habeas corpus additionally is not a "right". It is a procedure to enforce a fundamental right --not to be unjustly imprisoned.
As a procedure it is not self effectuating,. It requires statutory implementation. Over the years Congress has both limited and expanded the procedures governing granting a writ of habeas corpus. So have the courts.
Gonzales could have phrased his answer in a form more pleasing to the public. But he is not just "technically right". He is fundamentally right, and the principle underlying his answer is a greater defense of our liberty than a position that the Constitution is the fount of our rights.