If the demo was running on an A12Z, it would be the integrated 8-core GPU.
I don't know what kind of external PCIe bus exists for A-series chips (if they even have one?) so I don't think they could even add in a graphics card if they wanted to.
I would think future notebook / iMac versions would have a choice of SOCs with Apple or AMD GPUs, maybe connecting directly via Infinity Fabric in the latter case. A Mac Pro or xMac might be a different story though.
Specifications and benchmarks of the Apple A12Z Bionic GPU GPU.
www.notebookcheck.net