tangential and legitimate are
not mutually exclusive. the majority of the grievances are infact the latter; they are legitimate
what makes them also the former is that (and surely the reason for the rule) is that because these grievances are so commonly expressed when links are shared they create a near constant 'noise' in the signal to noise of each discussion, with signal being the idea or message intended to be shared by the poster rather than the medium it arrived in
a better way to discuss the tangent or the medium would be by submitting its own HN post. that way theres a better S/N ratio for everything
example: scientific breakthrough is shared on fbook
signal: any discussion about that breakthrough or its Nth order effects on anything
noise: any discussion about the fbook website, walled gardens, zark muckerburg, anything that would be on any link for that site regardless of the message inside the medium
those discussions arent illegitimate, they are better served by targeted discussion on their posts like..
some websites are stylized the point of illegibility
fbook website dark patterns
popups make alt F4
..rather than increasing noise across the whole of HN. this rule is closely related to another site rule: "use the best possible interpretation of the parent comment" because it too is a rule that is in place as an attempt to steer toward "curious discussion" (high S/N ratio)
( i will agree in this case the illegibility warrants a comment chain, holy smokes! )