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.