There's a definite tension between the rule of not accusing other users of being shills and the reality that there are quite a few shills out there. I think it a still good rule, but not because it's never right. Rather, the rule is good because the false accusations do more harm than letting some shilling slip by.
But the interesting question is: why did they write this article?
Is it just that the jig is up, and their one weird trick no longer works as well as it did? Did they get asked to cut it out by the HN moderators? It seems plausible given how many recent submissions to this domain appear to be auto-dead. Do they just think HN readers will forget about this article, and upvote their next bit of content marketing anyway?