Why is the content changing between refreshes not "(helpfully) bookmarkable"?
The HN front page (ie. "page 1") does that but it's a very useful bookmark.
You bookmark a link to the directory so you don't forget the directory's entry URL.
The use case the author is talking about is a different one: You are configuring a complex item in a shop, and want to bookmark the URL so you can save it, recall it later, share this configuration with someone, or compare it with a different URL.
In this case, you also would expect little details to change (pricing, descriptions, photos) but the structure of the state should stay the same.
It's very frustrating when you share a link to a product detail page, only to discover that all your filters and configurations have been lost.
Page 1 (a.k.a. the top few results with no pagination) has the property of being the selected top of HN, which is an interesting property in its own right, and what we're bookmarking. Page 2 doesn't have that property.
He probably wants to freeze the state of the page. Maybe he should consider saving it via ctrl s