I don't buy the language barrier reasoning either. There is simply no way that at a race of this magnitude, the race doctor does not speak English. If he doesn't, then a large portion of the peloton can be treated but not effectively. I can't imagine Le Tour taking such risks.
I also don't buy the the "I can continue because I'm a man" theory (that's not a direct quote, by the way). Sometimes riders do push through because they don't want to quit, but head injuries are different. You simply can't put on a convincing enough facade here to fool the team or doctor. Which means...
... I'm even more confused than I was to begin with. Bruyneel admits in the video that Horner was lying in a ditch shaking after the crash, AND that he didn't know where he was or what happened at the crash site or during his ride to the finish. Surely at some point, team management realised there was a serious problem? They must have known Horner was not in a fit state to ride, yet let him continue. How?
As for the race doctor, there must be an unenviable amount of pressure on him to let a rider continue. The rider wants to (even when he's spaced out) and the team certainly wants him to. But at some point surely you have to make a medical decision? One would hope the doc asked Horner the right questions to ascertain his true mental state. While that may not have been immediately apparent, if Horner spent a long time with him (as is alleged), then surely the doc must have realised something wasn't right? Yet he let Horner continue.
Assuming the doc saw Horner and thought he was fit to continue, it obviously became apparent on the ride to the finish that this dude was out of it. Why didn't the team communicate this to the doc? Surely the team has a duty of care to protect their employees?
I simply don't know how this happened, or for that matter how Boonen was allowed to continue. Is it simply that the race doctors and teams aren't used to dealing with such injuries? I don't recall a season where there has been such a large number of serious head injuries in major races as this one. Do they simply not know the symptoms? Having a rider seriously injured or die is not good for anyone. It's not good for the rider (obviously), it's not good for the team, it's not good for the sponsor, and it's not good for the race.
But this is just my opinion, made up in my room. I wish it was from my mother's basement, though. That would be a beautiful cliche!