It's still a full page refresh but with a page within a page. Making every comment a separate iframe would make the site very resource intensive on client and server.
I dont see why it would be resource intensive server side. In fact, it would be less resource intensive, since we avoid a full page reload and instead reload a single comment iframe.
As for the rendering engine, a iframe tag per comment is nothing compared to the pile of JS that's normally there on other sites.
Every frame is still a full HTML document that needs to be generated even if it just contains a single comment. This page has 370+ comments which would mean 370+ additional HTTP requests to load them all as standalone HTML pages. That's a massive increase in server load.
On the client, iframe tags are entirely separate browser windows and can use up considerable resources. It's like opening up hundreds of tabs and is far more intensive than running some javascript code which is quickly parsed, compiled and cached in modern JS engines.
