Any application can be said to be written in a single file if you ignore all of the libraries and just dump everything else into a single file, HN included. That's not a rational measure of simplicity, elegance or efficiency.
Sure having a modern infrastructure that is able to process thousands of requests per second is better than something that can only do a few hundred, but if you strip out a bunch of excess requests, you will process less per user, and thus reach the same level of scale on a less efficient stack.
The most important part is people paying you money.
Modern startup culture is full of inefficiencies + silly trappings (nice offices, drive to hire people, make things more complex, take more money from investors to stave off figuring out taking money from customers)
An entire app can be build in a single js file (with only one dependency... and it's 150 or so dependencies and sub dependencies).