A 6 hour window is less than 1% of a month. If you think HN has 100K MAUs then you should expect way less than 28,000 people to hit HN during that 6 hour window to see the link in the #1 spot. Even if the article was on the front page for a full day you wouldn't expect 28,000 clicks because fewer than that would be hitting HN on that particular day. Traffic isn't distributed linearly across the month but still.
Even then, I don't think this is a representative snapshot of HN users anyway. The average time on the site was 37 seconds. That's enough to work out what it's about and decide it's not interesting (to that person). As Codeamigo is a coding school, and HN has a lot of users who can already code, that's not surprising. It's likely that far more people read the comments first and didn't bother clicking after understanding what it is.
More importantly than all of that though, is that the link wasn't about Codeamigo. It was about starting it after being rejected by Codacademy. The interesting part of the narrative was about coming back from rejection, not the product itself. The fact 28,000 people clicked at all is amazing.