Can't confirm, but unless they've updated their tracking system, you'll likely receive cashback 99.99% of the time regardless of the card used. I've never not received Discover cashback while using Amex cards. As Lurker observed, it's entirely automated. If you clear cookies & cache (or, use private browsing) it should always work regardless of the card used.
The problem arises should the transaction for a large purchase not track properly. If you had used a Discover card, you can call and request they investigate and reinstate. Of course, if you didn't, then you're out of luck.
