The article says "'As soon as Amal found out she was pregnant, he hired former Secret Service agents to assess all his properties and make recommendations for improvement.' 
They claimed the star's mansion in Studio City, California was the most secure"

So they are moving to the "most secure" building, not moving because they believe the threat level is highest where they are. But this article has you convinced it is refugees and that I can't put 2 and 2 together. 

I've actually walked by Clooney's house in England. It is surrounded by a low fence and a large property. I can see why he might be convinced it is difficult to secure. The article says he gets death threats... is that the modus operandi of ISIS terrorists? Do they target international human rights lawyers, as is his wife? It sounds like he is worried about other threats since Muslims have not really gone after celebrities, have they?