Luxco (it hasn't been David Sherman for many years) is a bottler and rectifier, but not a distiller. Historically, at least, they have gotten their bourbon from Heaven Hill and at one point, stated that fact on the Rebel Yell web site.
Whether or not Heaven Hill bottles anything younger than five years is irrelevant, since they don't do the bottling for Luxco. They send them the whiskey in bulk and Luxco bottles it in St. Louis. Heaven Hill sells bulk whiskey at whatever age the customer wants.
Rebel Yell, especially as wheaters go, was always a little on the rough side. That was part of the idea. The bottle I have, of recent vintage, is nearly undrinkable, though not as bad as some Luxco Yellowstone I had the misfortune of buying. One doesn't know the details of Luxco's contract with Heaven Hill, but every producer right now is making their stocks go as far as possible because supply is so tight. Luxco obviously saw an opportunity to create a step-up product for the Rebel Yell customer, but their options were probably limited in terms of what kind of juice they could get.