The claim of direct causal claim is bold and probably more likely a contributing factor but can it really be the singular or even most impactful cause?
I think Haidt would certainly say this is the most impactful cause, but specifically his research has shown a causal relationship for this factor, not that it is the only factor. It's quite likely there are other factors that have some impact, but so far no one has made a compelling case for any other major factor.