On The Cards forms card games out of four parts (the deal, the aim, the card play and trick taking), with each aspect requiring the other three to make sense - if you don't have all four parts, you don't really have anything.
504 has an overall turn structure which the modules slot into to fill out various details. You can miss out the third module without really causing any impact on the game, and mechanically you can miss out the first or second module while still leaving something which is technically playable (if you don't mind stalling due to lack of money or playing indefinitely due to no end condition).
As a result, my view is that these approaches are different. However, this is a personal view, and I can see how people approaching the question from a different angle might see similarities.