It's hard to say if it was a promotional item or just a way to sell a few more Willow coolers.
The printed design doesn't have anything Holden in or about it and the Sandman font doesn't resemble Holden ever used.
But it is a cool item and would look good sitting with a Sandman on display. Certainly not the first one I've seen but your advantage is the Sandman marked box/packaging!