Nothing wrong with what he is saying. Even today IS/al Qaeda are not a threat to our existence. They threaten our lives, randomly.
Our own liberals are a greater threat to our existence.
But Carson is getting lost in the weeds for a campaign. It’s academic chat.
Back then, as now, the Mohammedan groups are always shopping for nukes. The may bribe one away from Pakistan or Russia and then hide it in a freighter and when it pull into a U.S. port like San Diego or Bayonne...Boom. The aftermath would include some degree of an economic meltdown.