This attack was contained about as quickly as it possibly could have been, apparently within single digit seconds, even if every person there had been armed. If he/they just wanted to kill Americans/Jews, they shouldn't have picked El Al for the location of their attack. A more likely place to have armed defenders doesn't exist, outside of the White House. :)
A similar attack on a local mall or baseball game crowd would be likely to go on for minutes before being brought to an end.
To that extent I don't disagree with you. It just wouldn't have helped in THIS case.