This is an anomaly that your explanation doesn't apply to though.

Notice that it has 50 points and is half an hour old. I count 12 items on the front page currently that have 50 or fewer points and are 1 or more hours old and yet remain there. These items are generally quality content (such as the tinfoil security post -- https://news.ycombinator.com/item?id=8086834 -- which sits at 37 points / 3 hours).

Your explanation of "5 votes within the first 30 minutes" completely disregards that this has as many votes as many other items on the front page, but in a fraction of the time period.