Why is it even possible to post content when it's not 9am-11am US ET?

There are other countries in the world. Those countries have people in them, and those people also use the internet. That includes HN.

The last reported stats by dang (years ago) put US users at about half the traffic.

https://news.ycombinator.com/item?id=40745850

Furthermore, there are people in the US awake before your time frame.