Does that mean HN runs in a single process and doesn't do any concurrent request processing? That's pretty impressive if that's the case.
The HN codebase doesn't have the overhead of parsing hidden POST fields (for example), but it does incur a massive RAM overhead to store all that information as closures.
A run-of-the-mill web app, on the other hand, would incur the overhead of passing state back and forth, and perhaps even store sessions on disk. But it might consume less RAM.
What matters is what kind of inefficiency you're willing to tolerate in exchange for what kind of benefits. RAM overhead is a smart choice if you want your app to be very fast and you can afford to use a lot of RAM. A different organization, however, might choose to incur a bit more code-complexity and slower execution in exchange for other benefits. It all depends on what your priorities are.