Yes,in the case of Massey's Landing if you click on learn more tab on GS campground in DE they are #1 listing and scroll to bottom of page it says GS discount and it has said that for some time. My understanding is the campgrounds that have top listings pay GS to have those listings and are supposed to offer GS discount.
Another campground that we enjoy quite a bit was a GS campground they enjoyed a top 5 listing but they decided not to renew with GS because they just didn't have enough sites available to make it a good financial decision for them to continue. That was fine with us and they were moved down to third page under Maryland campgrounds. We still go there and pay their current rate without a discount.
Like I stated before Massey's is the NEW thing on DE shore and no doubt they don't need to give discount they have no problem booking sites just don't provide them top billing on GS site if they don't offer members the agreed discount, they are getting centerfold, staple through the navel, front cover ad which I understand they pay GS a handsome sum, just don't make membership pay their bill. Hope this rambling makes sense. Not a great pontificator!