I largely agree, except it's easy to turn the topic back to the argument at hand. You just make statements about that, rather than about specific individuals, even though the other commenter did the latter. By commenting explicitly about the general and only implicitly about the personal, you convey respect and leave space for the other. It's different when the personal details are from your own life—relevant personal experience is fine.
We need this for discussion quality because challenging someone on their intimate personal details typically makes them feel wounded where it already hurt, prompting a defensive attack and a flamewar. Also, because we can't know from internet comments what was really going on in someone's life, it's more intellectually honest.
There are other things one can do to signal that someone is not being attacked. You can rephrase a statement as a neutral question. You can lead with "In my experience," making the statement about yourself. You can say "Of course I can't judge what was going on in your situation, but" and return to a general case.
I agree with you that when someone makes blaming personal statements as if they had general force in an argument, most of us immediately sense that something's off. But such statements come from pain and pain never responds to argument, only acknowledgement.
You're a fine HN commenter and I think you know all this already, but maybe the above will help clarify something for others.