I just tried using Ebay's 15% off coupon on a 100 coin slab holder, I knew most coupons don't work on coins but even slab holders won't qualify? It wouldn't adjust the price.
That coupon is for a select list of sellers and none of them sell coins or collectibles. You are thinking of one of the coupons for everything (which still excludes the coins and paper money category, but is good for all sellers).
Yes the discount would not apply as I assume the one you found was listed in the following category: Coins & Paper Money>Publications & Supplies>Holders
The only time the coupon works for slabbed coins is the coupon you get when you open your first eBay account. I used it to buy a slabbed Peace Dollar and it was a $50 credit for anything sold on eBay.
need more ebay accounts... usually have one at most times, sometimes they both would get the same promo, other times only one account will get one, but it's usually one a week. I even got one once for selling. I listed everything I had. Had over 200 in eBay bucks that month.