National Security Strategy of the United States of America [pdf]
whitehouse.gov
whitehouse.gov
I don't remember that being part of the Constitution.
The federal government has (at least in theory) limitations imposed by the Constitution. Within those limitations, though, it can operate on any theory of foreign policy that it wants, including that of "core interests".
This is sort of the whole point of elections. The government's core interests are not static. When the article says "always", it's from the frame of reference of the current administration, and it's pretty standard for whoever is the current administration to speak as if their stance has always been correct and will always be correct.
The framing here is that "America" always holds these things to be true, and any past discrepancy from that was due to bad leadership. This writing style isn't unique to today's administration, you can find examples from basically any government across history.
We were “always” at war with Eastasia