Reaction between Copper nitrate and ascorbic acid
It is always good to cite the link when you mention that you saw a redox reaction online. It is wrong anyway. 

Aqueous Cu(II) is not an strong enough oxidizing agent that it will convert organic matter into carbon dioxide. In other words, it is a weak oxidizing agent. Ascorbic acid is oxidized to dehydroascorbic acid and Cu(II) should be reduced to Cu(I). If you heat the solution strongly, Cu(I) may convert into Cu$_2$O. 

Ascorbic acid is indeed an acid, but there is nothing which prevents its conjugate base to behave like a reducing agent. Many organic acids can behave like that.


  I am really confused as to how they determined that to be the product. 


By conducting the experiment. There is no other simple way to predict what will happen in a complex redox reaction.