You'll notice all the demos of macOS Big Sur were shown using a Pro XDR Display, connected to hardware that was later revealed to be the MacMini+A12Z/Developer Transition Hardware. And during the Maya demo, it was a Pro XDR Display explicitly connected to the Developer hardware.
Which is interesting, because the Pro XDR Display can only be driven TB3 [1].
So Apple figured out the licensing for a TB3 controller chip to work in this design (which funny enough would be licensing that IP from Intel), or they are using USB4/TB4, or something else I'm not smart enough to think of.
Or they are bullshitting - even if the Dev kit wasn't ready to connect to XDR, they wouldn’t dare to show it. Which monitor would they even show it on? 10 year old Cinema? Glossy plastic Acer? :)
Which is interesting, because the Pro XDR Display can only be driven TB3 [1].
So Apple figured out the licensing for a TB3 controller chip to work in this design (which funny enough would be licensing that IP from Intel), or they are using USB4/TB4, or something else I'm not smart enough to think of.
1- https://www.apple.com/pro-display-xdr/specs/