I’m not an attorney but that seems to establish Federal jurisdiction over DC and lets it buy land for military bases (usually in places nobody can use for much like the pine barrens of Georgia and the Carolinas).
That really doesn’t seem to have much to do with Congress being able to declare war or having treaties supersede the constitution.
If you have time, look at this concurring opinion by Justice Thomas in United States v Lopez.
He said the U.S. Congress’ legislative jurisdiction is limited.
https://www.law.cornell.edu/supct/html/93-1260.ZC1.html