Clever use of a "HDMI-to-CSI bridge" gizmo. It allows the pi to capture the HDMI signal from the target PC using it's built-in camera interface. Allowing for boot-up / bios viewing which standard VNC doesn't give you.
According to the blog post, TinyPilot also uses µStreamer (from this Pi-KVM project) to achieve its low latency. The initial revision of TInyPilot couldn't get low enough latency using the HDMI over IP extender otherwise.