The DL Amex Reserve card does not actually provide an SC membership (it provides an SC access benefit). The distinction isn't very big these days (as SC memberships no longer provide access to DL partner lounges), but there are a few differences. Namely, you can access an SC when flying either DL or partners with an SC membership, but the Reserve card only provides comp access when flying DL metal (you can enter when flying partner metal, but there is a $39 fee). Although I suspect most people with the Reserve card have DL Gold or higher status as well and are able to use Sky Clubs when flying international partner flights based on their Skyteam status.