The product information page in the Apple Store says it comes with a "Radeon Pro 580 graphics processor with 8GB of GDDR5 memory".
The Radeon Pro 580 is built-in and non-upgradable. The eGPU.io website has a review that shows the internal components. There is no way for a normal graphics card to be installed.
The major benefit of the Blackmagic eGPU is that it is the only eGPU that lets you connect a Thunderbolt display directly to the GPU in the eGPU without extra hardware. Also, you can connect other Thunderbolt devices to it. These features could be added to normal eGPUs but Intel doesn't allow exposing the second Thunderbolt port of the Thunderbolt controller in eGPUs. The DisplayPort inputs of the Thunderbolt controller would also need to be exposed. It's an arbitrary restriction because some eGPUs do use the second Thunderbolt port to connect a second Thunderbolt controller internally, and also the Blackmagic eGPU exists...