So this post was on the front page from about 0300 to 0700 in California?

Yeah, I suspect the results are a bit off.

Nope, It is the estimated time-frame between which that post was the top post on HN.

It was on the first page for ~10 hours, starting around 10 UTC.

It was on the front page for a little less than 4 hours, beginning at 10:45 UTC. It was the #1 post just over an hour, from 10:50 to 12:00 UTC.

https://hnrankings.info/35553421/

(I'm a bit surprised that you got 30k visitors - that's more than I would have expected at that hour.)

That time of day suggests the American demographics are undercounted (not just US but Canada etc. too). On the other hand, your numbers are close to what I found when I did a similar analysis (already 5 years ago):

https://news.ycombinator.com/item?id=16633521 (March 2018)

https://news.ycombinator.com/item?id=16444556 (Feb 2018)