I think the point is that it's much easier to forget you're focusing on the speedometer instead of focusing on the road when the speedometer is physically displayed by a HUD right on the road. Especially if the speedometer keeps changing, since your eyes are naturally attracted to movement in your current field of view.

With a normal car dashboard, you're much more aware you're not seeing the road while checking your speed, and you don't actually see the speedometer moving while you're looking at the road, so it can't accidentally catch your attention.

Of course, none of this will matter in the vast majority of cases. But driving safety is all about the tail end, when you're slightly tired or when someone in front of you does something unexpected and maybe illegal, or someone jumps on the road - these are the times where accidents are avoided, and a HUD might well hurt rather than help for these cases.