I doubt he actually knows it, but there's a yuge difference between a formal declaration of war and an authorization of military force!!
I am sure he knows exactly what he's talking about. He wants a declaration of war against the terrorists. An unambiguous grant from Congress, unlike what the past two POTUSes have had. This use of force legislation is really semantics.
No, it is most definitely not semantics. A formal declaration of war hasn't been done since the 1940s. It triggers a large number of special executive powers with respect to the military, the economy, selective service, and many other aspects of normal civilian life. It also has a lot of implications for international law. That any candidate for president would even consider such a thing is massively unprecedented in the past several decades.