Granted, I am probably importing old thoughts of it being a sort of user provided style sheet.
You could say that Chrome is designed to tie the zoom level to the viewport but I wouldn't count on this behavior springing up from an underlying design and implementation rather than it being a design choice for the user experience.
That is, consider your network is down. You try to go to an address. It doesn't load, so you try another address, the page changes; but it is the same content.
That's what the GP comment said happened: the zoom level was the one associated with what they previously had set on HN, and they expected it to be the opposite, the default zoom level for the browser.
Is easier to see as broken by thinking of "how could I set it so that my browser's error page has a default zoom?"
In both cases, those are the browser supplying a resource representation, while still technically being on the resource specified in the navigation bar. The thing you're seeing is an overridden representation of the server's response. (Which, in this case, just happened to be "no response.")
It's almost exactly the same as how the server sending a 304 gets the browser to load the document from cache. The server's actual response was a 304; but the browser's representation of that response is the cached HTML DOM it had laying around from the last 2xx resource-representation it received "about" the same resource.