Why not both?

Clearly some people are attracted to far-right ideals without being in the petite bourgeoisie and some people who are in it are even repelled by such ideals, plus there is the whole domain of the "highly educated" who tend to be less attracted to far-right ideals and yet are often generally more prosperous than most shop keepers and similar and some even work in similar business structures (such as Architects with their own Studios or Lawyers with their own small Legal Practices) hence would count a that kind of petit burgeouisie.

I would say that it's a mix of what you point out (so people's petit burgeouis status or, as I would put it: "people who have just enough material wealth to think they're wealthy but without the education and worldliness to understand that they're nowhere close to real wealth"), certain character traits such as one's level of Empathy and Self-awareness, one's breadth of life experience (not in term of years but of how many different things one has done and seen and kinds of people one has met, which would explain why city people are less likely be attracted to the far-right that more provincial types) and one's style of thinking and practice with things like analysing real world situations and trying to solve real world problems (which would partly explain the effect of Education, the other part falling into breath of experience, specifically in the form of how much information one has the tools to understand).

This is without even going into the environment one grew up in and lives in: sometimes that kind of thinking is so widespread in one's family and were one lives that showing the social cues of far-right belief and even believing it is a natural element of fitting in if only for one's own protection, similarly to how people tend to be religious when coming from a religious family and living in a religious community.