How much do I need to worry about an accident such a Cedric Gracia's and What happened in this incident?

That was a freak accident. Very rare. Still a good idea to learn from the video though. He was very lucky that he had people around who knew what to do.

People can hurt themselves walking across the street. I have fallen hundreds of times on my mountain bike with no injuries more serious than a broken rib and a broken clavicle. Life threatening injuries and death on bicycles are rare occurrences. You stand a higher chance of dying in a car crash than you do riding a bike. Do you plan on not driving or being a passenger in a car?

Looks like he is going fast down a fire road or something like it. Hit a rut, handlebars turned 90 degrees and he impacted his leg area. Probably a bit of being careless because it was an easy road.

Unluckily, if he separated from his bike he probably would have been fine although a bit sore.