Except you shouldn't have to anticipate some numb entitled prat watchj traffic around him stop and not realise why and do the same.
The car driver can only deal with what he/she sees. They are being given right of way by a courteus motorist and essentially have to place some faith in that they can take that right of way safely. So really, it's the motorist who stopped to let them turn-in that should have taken some responsibility as they would have, or at least should have, had sight on both parties in this situation.
That being said, the cyclist was in the least predictable situation and should have reacted to the situation around him, not just thrown himself past a side road without being aware of why the vehicle alongside has stopped. People don't leave gaps for no reason.