Not OP, but as someone who holds this view (who also used to engage in the practice): a lot of armchair psychoanalysis is based less on a genuine understanding of the other person's life and circumstances, and more on the assumption of what their life and circumstances must be combined with a surface-level knowledge of psychoanalytic practice.
Armchair psychoanalysis ostensibly seeks to understand the subject of analysis, but rarely makes the effort to first understand the subject on their terms or in a way where they can articulate their own experience; instead, someone usually has their presumptive conclusion about the subject in mind ("they're just doing this because they haven't gotten over being bullied as a kid" or whatever), and tries to wrangle the limited information they have about that person into their conclusion.
Armchair psychoanalysis ostensibly seeks to understand the subject of analysis, but rarely makes the effort to first understand the subject on their terms or in a way where they can articulate their own experience; instead, someone usually has their presumptive conclusion about the subject in mind ("they're just doing this because they haven't gotten over being bullied as a kid" or whatever), and tries to wrangle the limited information they have about that person into their conclusion.