There has not been a formal declaration of war by Congress since WWII, but I believe there are plenty of court decisions holding that conflicts like Korea, Vietnam, the first and second Gulf wars, and what the US has been doing in Afghanistan since 2001 are wars in the sense that matters for treason, since Congress passed resolutions authorizing the use of US military force, even though they weren't called declarations of war.
Also, a person can "levy war" against the United States even if the United States does not formally recognize what is going on as a war. "Levying" war is broader than "fighting against in a formally declared war".
And, finally, the other clause in the definition can apply at any time, even if there is no US military action going on at all.
Is that reading backed up anywhere legally, that the definition of an Enemy only applies during a state of war from an official declaration of war?
Here, here's a related article by a law professor at my alma mater: https://www.washingtonpost.com/opinions/five-myths-about-tre...
The United States has not been in a state of war since 1945, so very few people alive could have possibly committed treason, unless they did so as a child. No person under the age of 75 could have committed treason.