From the blog you linked, the number of interest is 18k. 12k are only those with HN referrer headers. In reality, many setup strips that header so you can't track it exactly right. The author did mention they averaged 50 views before.
A big part of it are reposts. From my own submissions, posting to HN resulted in tons of different origins. Public ones like reddit, twitter and private ones like newsletters, dashboard & chat messages. You'll also be surprised by the wide variety of clients people use to access HN.
They also used Google analytics to track the numbers. Most people in HN block it either through the browser or an extension [0]. In reality it's probably double the traffic.
Don't forget to account for scraping & crawling bots. That's another big source of traffic that the author didn't track.
[0] https://plausible.io/blog/google-analytics-adblockers-missin...