[noumenized]

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.