In general I think that article is completely flame bait - but I’ll share my opinion on some of the general underlying points.
My site Teenormous makes the majority of it’s money with affiliate marketing so I have some experience in the space. The discount / voucher / coupon code sites have always been a somewhat contentious topic within the affiliate marketing space, especially among merchants. As a result, I’ve always stayed clear of trying to target “[merchant] coupon” type of traffic.
I do believe that coupon affiliates for the most part swoop in and “steal the sale” that was already in progress. It probably happens a lot more in some industries (such as retail) then I would expect it to happen for smaller SaaS companies like many on these forums. The coupon affiliates I think would say they are providing the value of finding the deals - however pretty much all of the coupon sites claim to have codes, even if they don’t, and they will still set their cookie. I’ve even seen codes listed for Teenormous which is pretty funny since we don’t directly sell anything. As a result, many merchants that have affiliate programs are either rejecting coupon affiliates, or giving them a significantly reduced commission.
My personal opinion is by adding a discount / promo code / coupon box on your check out page it will interrupt your checkout flow for a measurable percentage of your customers and cause them to leave your site to go find one. Hopefully they will return, but I would imagine a lot of them may not for a variety of reasons such as interruption, frustration with fake coupon codes, and a lot of those coupon sites also cross promote competitors as well.
I’d be more inclined to post the promotions for your site directly on your site if you want to reward the customer. This keeps them on the site, and make them feel like they are getting the best deal.