To be a federal installation it would have to be formally commissioned. It never was.
I’m not sure how you arrived at that hair-splitting conclusion. Even though it was still under construction it was inarguably federal property and it was manned by federal troops.
As I'm certain you already know, there was an 1836 South Carolina law:
"Resolved, That this state do cede to the United States, all the right, title and claim of South Carolina to the site of Fort Sumter and the requisite quantity of adjacent territory..."
In a sense, it is equivalent to the treaty with Cuba by which the US leases Guantanamo, a treaty now rejected and denied by the Communist Cubans.
If the Cubans assault US forces in Gitmo, that is an act of war, pure and simple, regardless what the commies say about it.
Same thing with Fort Sumter.