You missed this quote:
“Maybe he’s a super guy,” Lombardo said before catching himself and calling it the wrong word. “You know, a super yay-hoo that was working out all this on his own. But it would be hard for me to believe that.”

The investigators are looking into if he had some help and have doubts that he could pull this off alone.  Either in the training, the set up, the surveillance cameras, the escape plan, the loading of the ammo, the use of bump stocks, ...  there is a lot of advanced things here that imply some training and/or assistance.  So I know you think this is an open and shut case, I think the investigators are far from sharing your conclusion.  In many ways, this guy is a perfect person in that he had no real close friends, kept to himself, had little social media, moved around a lot.  That is why the girlfriend is so important.  She is the only one who seems to have been close to him.