I can understand sitting tight inside your house if there’s one man threatening to come in. But, with a large group of men swarming the property, I can understand a homeowner wanting to keep them away from the house, instead of waiting for them to try to get inside. It seems he was caught in a situation in which, no matter what he did, he was going to have trouble.
Why would he want to increase the defended perimeter? Quite the opposite, the defender(s) want to use natural obstacles (such as walls) to help them out.
Another view of the same: if you defend a larger area then all 20 attackers can spread out, pick their own positions and do unto you whatever they want. But if you defend a smaller area, like a house with two doors on the ground floor, those 20 attackers will have to stand in line to get inside, blocking each other from any efficient action against you, and presenting themselves to you one by one, allowing your AR-15 to cycle.
Besides, as the guy in the story is now hearing from his lawyer, shooting at a gang member within your house is very much different from the same shooting at the same guy just outside of the house. The difference is measurable in years.