I have often seen this statement, and it's simply wrong. The Declaration of Independence is an exptremely important and formally binding document which was debated and voted on during the Continental Congress in Philadelphia. (I refer those who doubt this to the recent biography of John Adams).
It was formally adopted by Congress, and was a the proximate cause of the war between the United States and Great Britain.
You may mean that the Declaration is not the framework of our laws, as the Constitution is. That is true and immaterial. The Declaration is not a letter, essay, newspaper article, or the expression of a single individual. It was drafted initially by Thomas Jeffeson, and each phrase was debated and voted on by Congress. In the end each member of congress signed it, giving it the force of law. It's what the subsequent war was about.
Love and peace.
"WE, therefore, the Representatives of the UNITED STATED OF AMERICA, in GENERAL CONGRESS, Assembled, appealing to the Supreme Judge of the World for the Rectitude of our Intentions . . ."
Tell me, if such language were drafted -- for any reason whatsoever -- today, would these words be unconstitutional?