HN may be optimized for current discusions, but is also a repository of knowledge; I frequently land on years old threads from Google searches. For older times, an explicit ISO timestamp has at least two benefits: 1) I can compare it easily to other dates (e.g. is that comment older or more recent that event X, or software version Y), and 2) time of the day is a half-decent proxy for telling whether the US or Europe was awake then.

And while you can justify the inprecise stamp on HN, there's no excuse for GMail and a score of other messaging software doing the same. When I'm looking at the date of an old e-mail, I almost always need the exact date and time. "2 weeks ago" is useless. "2019-09-27 08:43" tells me it happened before work, on a day after an important meeting, etc.

ISO8601 date/time can easily be transformed into the fuzzy format HN uses, but not vice versa since the fuzzy format is lossy.

If HN sent dates to the client in ISO8601, the client could then transform that format into the format of their choosing with client-side javascript (either a userscript, or delivered to the user by HN.)

It's hard to be sure this isn't satire.

You mentioned 'for HN' twice in the same sentence.

Your 2nd, 4th, and 5th sentence all try to assert the same thing - older comments are less relevant than newer comments - despite there being no evidence, anywhere, that this is the case. Your 3rd sentence is partly true, though there's some historical record aspect to HN comment threads that you're discounting.

Your second paragraph is a non sequitur. Sites that correlate points in time with timestamps are ubiquitous, and there's rarely a compelling case for using wildly different approaches to displaying same.

They could easily do it the Reddit way and still provide a timestamp but keep the simplicity and relative time format. Just have it pop up the timestamp on mouse hover. That's still very simple and can be effectively done using just CSS.

You don't even need any CSS for that — the title attribute alone would be sufficient.