there is no detailed explanation of how the thief could find out that the tourist has bitcoins, however, bitcoin ownership cannot tell it to other people because it is very sensitive and can provoke crime.
I think that doesn't need further explanation, the article says his a blogger, entrepreneur, and investor? Well, that along explains why and how these criminals found out he has been storing Bitcoin.
I guess we are all aware that robbers may forcefully stole our Bitcoins in front of us and can get rid of the transactions to be tracked. So, in the light of this event, we are all warned again not to show your crypto assets publicly.