I doubt that the author can find those words in the Constitution since they are of course in the Declaration of Independence but not in the Constitution.
The Declaration of Independence is one of the founding documents of our nation, and is frequently quoted in Supreme Court and other federal court decisions.
It certainly illustrates the intent of our Founding Fathers, and is used in that regard.
The words aren't in the Constitution, you are correct, but the sentiment certainly is. I could probably cite you a hundred passages enforcing the idea that all men are created legally equal in the Constitution, even before the passage of the 14th Amendment (which obviously was not an issue in D. Scott).
And there is also the three-fifths compromise in the constitution which contradicts that statement too. (Not to mention voter qualifications in the early 19th century).