That is explained by this sentence. A car which is on a collision course at Ipley Cross with a cyclist who is obscured from the driver’s view by the front pill

That is explained by this sentence. A car which is on a collision course at Ipley Cross with a cyclist who is obscured from the driver’s view by the front pillar will approach the cyclist from behind. So if the cyclist doesn't do a shoulder check, the cyclist won't see it. If they do do a shoulder check, the car will appear to be stationary to them. (It is surprisingly harder to see an object with no apparent motion.)