It refers to the fact that people get rich from bitcoin if they don't fall off their bull by:
Losing their coins e.g. forgetting a password, throwing away a laptop.
Having their coins hacked from their computer by a Trojan or the mentioned attack.
An exchange loses them or shuts down.
Due to greed you wait it out and bitcoin plummets to zero
Due to greed day trade your stash into the ground.
Due to fear sold your 1000 btc at 10$ each back in the day.
Etc.