This is a 2015 paper. I think there has been some evolution on the thinking about whether a history of emotional trauma is necessary, or even if psychological conflict and repression of feelings is necessary. I think that evolution was necessary because studies just did not support those criteria in all of the people with functional disorder [add your favourite synonym] labels.FND is mainly referred to as a 'software' issue as if it's too complex for anyone other than the researchers to understand. Yet, in this paper, it has been clearly explained as symptoms resulting from psychological conflict.
As to where that then leaves the functional disorder concept now, well, of course it's a mess. Basically it's the catch all for when doctors can't justify another label.