I think the important thing to keep in mind is that reaction and fascism are not individual endeavors. People can end up in them for a variety of reasons but the material conditions and ideology of a society are far more important than an experience of trauma. And critically, you can react to trauma in a lot of ways, depending on your environment and the superstructure in place around you.

The reactionary political project is fundamentally a rear guard action against the left, I find this part of Corey Robin's The Reactionary Mind pretty convincing. And in the case of Weimar Germany there was a concerted effort on the part of fascist and reactionary liberals to turn the public against socialism, with propaganda and policy and political maneuvering. But in other circumstances (the Russian Revolution comes to mind) trauma was used as a fulcrum to help move society left very rapidly, through organized efforts by a different ideological group.

So I guess I would say they're right that trauma, especially society wide like a devastating war or a pandemic or whatever, create a rupture that gives these ideas an opportunity to grow. But that's not a determined outcome and it can also cut the other way.

Parenti talked about this, https://redsails.org/against-psychopolitics/

I suspect that trauma is being a tool of reactionaries to create more reactionaries. Imagine starving and having the media tell you it's because of X-men (lgbt+, other race, other religion, ect). European elites told their citizens that god was punishing them because the Jews in the 15th(?) century, so the Jews were expelled. Over time the elite began to realize they could escape judgement for poor leadership and remain unquestioned as long as there was an "other" they could blame and punish. The leadership did not wish to be judge, so the leadership found a scapegoat.

During the rise of industrialization through capitalism, they need more scapegoats, so they found more. By indicating to people that wealth and private property were signs of good morals, they were able to claim a moral high ground in the eyes of the traumatized. So by spreading this logic in areas where the population hadn't yet recovered from a shock, they are able to spread fascism. Lots of countries have dealt with trauma without turning fascist. Trauma can absolutely help create fascists though.

Trauma to create fascism is basally a few steps:

Step 1:

Man made trauma, economic, drug wars, colonization, slavery, cyclical capitalist crisis, war, disease

Step 2:

Blame the poor, or other, and flaunt wealth, so they know that the holder of the wealth is the good guy, not a thief.

Step 3:

Tie aid for recovery to demonstration of fascists ideas so that only fascists can thrive.

I do know that quite a few studies have been done on the relationship between right-wing authoritarian + religious fundamentalist beliefs and head trauma. There is a very clear link between the two. People who have suffered major head injuries oftentimes end up as fascists. I'm not sure why.