the Pentagon is the name of a building (pretty much a very large bikeshed). I see the actual agency is named by the author as the Defense Department and one of the officials in question is a Defense Secretary. Interestingly, the bikeshed itself has its own spokespeople.
The Pentagon is a synecdoche for military leadership in the United States.
there is also a figure of speech used soo often, called metonymy.
The White House is also the name of a building.
News sources have been using both building names (and several more I can think of off the top of my head) as short hand for the people who work inside of them for my entire life.
It’s called a metonym. Look it up.
"The Pentagon" has been the nomenclature for DoD / DoW for decades. Everyone knows what it means.