Maybe, but it also depends on the level of physical evidence obtained when they processed him after arrest as well as from the school. Did they find abandoned facemask and clothes and whose DNA is on it? Whose prints are on the rifle.
This school has security cameras everywhere, so if there are multiple shooters, they would have announced that and been looking for them also.
It’s easy to impeach an eyewitness as mistaken, but it is much harder to impeach physical evidence and video.
20 minute gap in video as well. Some audio missing.