This is an act of the Confederate Congress.
It does "recognize" war with the United States.
It authorizes acts of war against the United States.
This and several other acts of the Confederate Congress authorized war and war-powers to the Confederate government.
So there is no practical, legal or logical distinction to be drawn between Confederate actions and any other formal Declaration of War.
And here's the important point: the United States made no invasions, assaults or attacks on the Confederacy -- no Confederate soldiers were killed in battle with any United States military force -- until after the Confederacy first started, then formally declaration of war on the United States, on May 6, 1861.
You said: “This is an act of the Confederate Congress.
It does “recognize” war with the United States.
It authorizes acts of war against the United States.
Yes, but there is no vote of the Confederate Congress declaring war.
Say what you will, you have no fact.
It does not record a vote of the Confederate government for war.
The Act to which you refer is known as "An act recognizing the existence of war between the United States and the Confederate States, and concerning letters of marque, prizes, and prize goods."
Your date of May 6 is three weeks after President Lincoln started the war, as determined by the US Courts.