Steam lies to me :'(

Gooby pls.

Dunno about the specific coupon but might be that it predates the release of Foul Play?

Kill them. All of them.

I'm assuming the coupons are similar to subs where they have a list of included subs/apps that they can be used on and since the coupon existed before Foul Play, it's not included.

I would contact Steam Support and also shoot a message to Devolver asking about it. If you follow the link that shows what the coupon can be used on, it's clearly not listed so it shouldn't have been a huge surprise that it didn't work.

Ok.

I had a coupon for Serious Sam 3: BFE with 90% off but in my basket it was only worth 75% (calculated it). So I wrote to Steam support and guess what happened? Right, they corrected the imagine without any further word to me. :P What we learn from it: The coupon is a lie.
PS: I'm not the necro. x)

Pre-order Shadow Warrior now and receive the exclusive Zilla Enterprises Z45 katana in-game weapon and a Steam coupon redeemable for 75% off any current Devolver Digital game or Flying Wild Hog's "Hard Reset" on Steam.

Foul Play wasn't released back then so it was kinda obvious that it wouldn't be included, I tried it myself though, you never know ^^