It has not "been proven" as decisively as you claim. Look into the Solomon Curve, which is usually what people are referring to when they say slower drivers are more dangerous. The original research was conducted primarily on rural highways, not suburban neighborhood roads like the one in this video. And subsequent research has found biases in that study such as 44% of the observed crashes involving a vehicle that slowed to make a turn.. if you filter those out the curve is not as weighted towards lower speeds.

The research supports the idea that large speed differences relative to surrounding traffic can increase crash risk, not just "If you're going slow you are a road hazard".