The external GPU is in an enclosure and that enclosure doesn't have to have a GPU in it. Most of the Thunderbolt 3 to PCIe enclosures support other PCIe devices.

Think of a small case with a power supply unit and a single PCEe slot inside. You plug any regular (internal) video card of your choice in it, and connect the case to the main computer via TB3, making the video card available as an external video card. That's a TB enclosure. These things has been for awhile, at least since TB came out.

It's not called eGPU enclosure because technically it can contain any other PCIe card, though it is almost exclusively used for video cards as other devices usually can be powered by TB cable itself.