I don't think I've ever heard him suggest it was not a big deal, or that it wasn't a dark day. He's not whitewashing or downplaying it. He's trying to logically explain it. There is a difference.
His biggest thing, especially once he got swept up in it, is that it appears there were a lot of weird little things from that day that don't add up. If you read his account of the day, he pointed some of it out right away. Like Trump supposedly inciting the crowd, but the audio system at the rally was absolute garbage and it was hard to hear anything, so how many in the crowd even heard what he said? Or some of the interactions between protestors who appeared to be well-prepared and organized. Or, as he's gotten further down the rabbit hole with the video, cameras that were focused on a spot for weeks suddenly pivoting to a different angle right when something important is happening in that spot.
What he's tried to do is go back, review video — thousands of hours of it over the past year, from hundreds of cameras — and connect the dots on those anomalies. He's said some conspiracy theories were easy to dismiss once you see something on video from a couple of different cameras. Other theories gain more credence. Some narratives on both sides get blown apart.
No one else has been either able or willing or motivated enough to take on the gigantic task of sorting through that mountain of video evidence. He's obviously got all three of those things going for him.
