SPAs use Ajax to sync data with the server, but the templates/assets are on the client already. In a regular webpage, Ajax would have to pull the templates for the new components as well.
I think this is what Gmail does, though it still takes forever for the initial load. Anyway, I can see this becoming a complicated mess very quickly, and find Ractive/React to be much simpler solutions.
As for HN, I doubt anyone comes to HN for the design. PG and YC's brand helped create a solid community with good content and good discussions, which is what keeps bringing us back. But let's call a spade a spade - the UI design here is a joke, even more so considering that its target demographic is the tech community.
