I don't think Jetpens makes any pens themselves, although they are a retailer and sell all of these pens so it's definitely advertising. However, Jetpens in generally reputable and their recommendations are generally good. HN typically likes posts about tools, so it's not too surprising a good tool recommendation post is popular here

JetPens does a great job in teaching you about different pens, and recommending them too.

They’re the real deal. Their social media and YouTube people should get raises haha. I have learned a lot from their videos.

This is a straight advertisement, it's bizarre that no one seems to know or care.