I've been eagerly waiting for this video since I saw the tweet of Linus playing Xonotic in February!
Not all of them do; you can see in some parts of the video that Linus is holding two differently-sized CPU modules. The smaller one is Sforza, which is what Blackbird (the board Linus holds up) and Talos II (the dual socket system he plays Xonotic on) use; it has four channels. The larger module is either LaGrange or Monza, both of which do have eight channels, but are more expensive.
Technically, all three modules are using the same silicon, so the extra DDR4 controllers are still physically there on the die, but are probably disabled on Sforza. Even if they were functional, the Sforza socket, LGA 2601, has much fewer pins than the Monza/LaGrange socket, LGA 3899. I imagine some of those extra 1298 pins are for DDR4.
The RCS company/community wiki at has a pretty good explanation of the different modules if you're curious.