Using federal law enforcement officers to protect federal property could not be more obvious. Here is what the statute says in regard to adding National Guard. Notice it is his decision, and not the opinion of a state governor or some judge. There is nothing in the statute which says the Congress can take away his authority.
A Federal statute cannot be properly interpreted to repeal or amend any provision of the federal Constitution.
U.S. Constitution, Article 4, Section 4.
The United States shall guarantee to every state in this union a republican form of government, and shall protect each of them against invasion; and on application of the legislature, or of the executive (when the legislature cannot be convened) against domestic violence.