The article ended and then there was an advertisement in italics, I didn't read the article as an advertisement for their book as much as just a comprehensive slamming of NPS.
Which is a terrible metric that pretty much nobody should use.
I don't know if this tactic applies in this case. Jared Spool is a very well-known UX expert, he's also been ranting about NPS for ages. It feels more like the other way round, he wrote the post and attached a sales pitch, rather than (as you say) attaching a post to the sales pitch.