Cool video and I watched it several times. According to the video, there was a hole in line to the right of the aircraft. When they got the hail, there still was a hole again to the right of the aircraft. That hole closed up after they had past.

I am sticking to my original thought before the I watched the video. I still think that Delta, AKA Northwest, thought they would be on top of that Thunderstorm that they penetrated. If the video is correct, they could have flown through the hole that was closing up had they gone farther to the right.