Now that I look at it again, I think you're correct. It wouldn't make sense if this wasn't projected onto some kind of canopy over the driver.
And when I look at this again, I think it's a camera from inside the canopy, rather than the cockpit being open. You can kind of see there being a seam around the entire cockpit, back to the roll bar, for some sort of covering to be secured into.
It does make more sense if it is closed-cockpit.
Edit:
You're absolutely right.
Also, looking at the HUD, 397 km/h in 6th gear is quite impressive, and really shows the potential power output and aerodynamic efficiency of the X1. Let's just assume that the X1 has at least 7 forward gears (like a current Formula One car), or possibly even an 8- or 9-speed gearbox (but that would add weight).
If you look back at the 3.0L V10 era, ending in 2005, the cars were pushing 360 km/h or so (roughly, from memory) at the end of the front straight at Monza, in low-downforce configurations. If the X1 is achieving 397 km/h in 6th gear, while still quite clearly accelerating, I don't see 450 km/h in 7th gear all that unreasonable to achieve on a fast circuit.