The ground staff did what they are supposed to do, as they are not supposed to enter the field when the players are out, even if there is someone who decides to run on to the pitch.
And as Martin has said, the stewards at Saints are some of the best in super league but if someone wants to run on to the pitch they are not going to stop them unless you have stewards stood 2 feet away from each other right around the perimeter of the pitch