the former GraphCMS was built on top of Graphcool, with the new system, we also move to Graphcool's new product, Prisma, which is a setting we enjoy a lot. Prisma runs at the core for CRUD and then we add an additional API layer to it to add the features we think make sense in a CMS context.
The value add depends on perspective. If you are looking for a tool to build a content database, then GraphCMS brings definitely a lot to the table. While you get the content management interface and tools, we add just enough opinion to the APIs that makes sense in a context of building a content database.
Graphcool is also not actively maintained anymore as the team is now focusing on Prisma.
The Demo-API on the landing page is from the former stack. We will soon add a new example to showcase all the new API utils.