I think Cayde had Banshee secretly make it for our Guardian. During the quest line, the quest descriptions show Banshee hinting that it was a special order. The Grimore also shows that Cayde was trying to keep it quiet. While it could be that using his Vanguard status to commission special request could be inappropriate, it seems more likely that it was to be a gift. To us. My theory is that Cayde knows that he won't be leaving the Tower anytime soon. So to honor his friend, he is giving us his Ace against the Darkness.