I've seen a photo of the moment that the ball is over the line and it certainly appears as if the referee is looking in a completely different direction. It's possible on the basis of that photo that he genuinely didn't see the ball crossing the line, at which point he would refer to his assistant for confirmation.
The linesman, however, seems to be looking right at it and doesn't seem to be having his view blocked by anyone so how he can miss that is mind boggling. Even the assistant on the opposite side should have been able to see that it was over the line.
Utterly bizarre.