I've not worked out when firefox reader view is visible and when it isn't, but for this blog it isn't, which is a shame because I find the chosen font unreadable.
In a nutshell, the paragraphs (<p> tags) do not contain enough text to contribute to the readability score. Reader view heuristics rewards paragraphs of at least 140 characters.
simple != simplistic. "most minimal" != "minimum".
the 'just read' chrome/edge extension does work, and vastly improves readability of the produced html.