One cause was the low level of social trust.
[…]
One [other cause] was a deterioration of physical health,

In which context, the rest is mostly redundant.

The other cause was a decrease in average cognitive ability. Fewer people could master the skills of numeracy, literacy and budgeting that are so essential to economic activity.

The authors think reading is a great cognitive benchmark. It’s not a good sign.

The resulting fall in cognitive ability can be seen in DNA retrieved from the human remains of that period.

And then it rolls into laughable quackery.

Isn’t a combination of social connections falling apart on a large scale and health problems (poor nutrition?) potentially sufficient to cause a general decline on its own?

I think the picture of Roman Empire collapsing into “Dark Age” by Kevin Crawford here can be extended backward in time, since Rome did not collapse overnight but was falling apart gradually.

https://old.reddit.com/r/SWN/comments/gtpelj/rational_behind_post_scream_inability_to_travel/fsdmnoe/?sort=old

As «giant iceberg of trade, production, social compacts, skilled personnel, and in very partial part of simple technical data» melted away little by little, this does not require completely different mechanisms for “before” and “after” it fell to pieces.

The failures increased, all sorts of things were failing. The response to immediate problems drained resources, while selection for pessimistic hedging and “turtling” strategies reduced risky investments (of all sorts) into future surplus (Taleb wrote about that part a lot). This obviously can form positive feedback loops. So all those effects draining “non-essential” capabilities only increased — until there was not enough left to keep the entire thing together, and it fell apart.