The display uses a mipi interface instead of a edp. This is because the screen isn’t completely custom and likely used one from a manufacturer similar to the switch oled which also uses a mipi display.
If valve sold more units it might justify a completely custom solution but this is still way better than what people had to do to get a better screen before.
You can just open the Deck and look. It's not a 40-pin eDP connector, it's a flat ribbon, exactly like a tablet display. Also the natural orientation is portrait, which you can see if you turn off the boot splash.
If valve sold more units it might justify a completely custom solution but this is still way better than what people had to do to get a better screen before.