Yes, the "discount" is not a cut that the distiller is making, it's the lack of taxes. So while it is a reduced cost for the purchaser, it's not a discounted product.
I'm very skeptical about the idea that companies are providing different product under the same labels. There are some whiskys available from some distillers (Bowmore and Bunnahabain come to mind) which I've found are not available through retailers, but these are specific blends and are unique in name and labeling. I've also only had the opportunity to try the Bowmore's Duty-Free-Only whisky, but I thought it was quite good.
That said- and this may seem to discredit my first statement- I've had the liqueur "Irish Mist" which was actually purchased in Ireland and found it enjoyable, while I've never cared for the Irish Mist sold in the US. But that would be a distinction of bottles made for export vs. domestic sale, so maybe the Duty Free variety is either export or domestic? I sincerely doubt that a 3rd variety, specific to Duty Free sales, exists.