Angel's Envy - Is Bourbon Aged in Port Casks Still Legally Bourbon?

I recently received a bottle of the Angel's Envy Kentucky Straight Bourbon Port Finish - which is almost as nice as their Cask Strength. For those unfamiliar with the brand, it was started by the late Master Distiller Lincoln Henderson and his son Wes. Henderson helped develop several major bourbon brands such as Woodford Reserve, Jack Daniel's Gentleman Jack, and Jack Daniel's Single Barrel and is an inaugural member of the Kentucky Bourbon Hall of Fame.
Angel’s Envy Kentucky Straight Bourbon is produced using the legal requirements for Bourbon - over 51% corn, aged in new charred American Oak, as well as meeting a few proof requirements. It is entitled to be labeled Straight since it has aged more than four years, in this case at least six years. But then the whiskey is aged between three to six months in used 60-gallon ruby port barrels made from French oak. The result is a complex whiskey: vanilla, toasty honey, candied citrus, and plenty of nutty character. But, with this final finish - can the product be legally labeled a Bourbon?  Any ideas out there?