The XDR is probably being driven by the M1 GPU instead of the Blackmagic eGPU. There's nothing interesting here except that the DisplayPort signal is able to pass from the M1 Mac, through the Blackmagic, to the XDR display. This leads me to the following idea:
Can you connect more than two displays in macOS to a single Thunderbolt port using a Blackmagic GPU? You need four HBR displays (1440p60 displays or any displays set to DisplayPort 1.1), an Intel Mac to drive two of the displays, a Blackmagic GPU to drive the other two displays, and three Thunderbolt docks to connect the four displays. Of course, an eGPU can connect as many displays as it has outputs. The trick here to be tested is that the 4 DisplayPort signals are being transmitted through Thunderbolt. This goes to the feature set of future USB4 controllers that may have more than 2 DisplayPort inputs (currently, Thunderbolt controllers only have max 2 DisplayPort inputs).