I'd love to hear from any other user who has an unsupported Mac with TB3, as to whether a Blackmagic eGPU gets them 6k. I just tested on my MacBook Pro 15 inch (2016) with Radeon Pro 455 2 GB graphics. When I connect the Blackmagic Pro eGPU, I also only get 5k from the Pro Display XDR.
With Blackmagic themselves unable to stand behind this, and two machines not working, this could mean the support document is wrong. Maybe they meant that anyone can get 5k from the Pro Display XDR with a Blackmagic, but not 6k (even though it explicitly says the opposite).