The reason he is being taken to Confed cup isnt as much how impressive he has been, but that the U-21 doesnt need him, hence he can be taken by Lippi and given a chance. Euro 21 is a rated tourno, and Casiraghi wouldnt have stood for any of his regular players not being available, was never an option any of them would be called up, hence Motta is with this squad instead of Confed squad. Santon just played 1 friendly game, is born 91, so he can play in the U-21 in coming years with whatever tournos that comes up etc, while likes of Motta and Criscito (starting fullbacks, with good backups like Pisano and De Ceglie, so rather stacked) will play their last games for this level (indeed, they are 5 years older then Santon), and they would bench Santon's ass most of the tourno.
So as I said, these are the main factors of why he is called up. Him being this young and this stage of his career is rather good, but if there wasnt an euro 21, his a dozen of decent games wouldnt have been enough to get himself a call up, other Azzurrini players would have been called up before him.
P.S Another thing though Santacroce doesnt make EITHER of the squads, wtf.