This whole thing feels like an advertisement. Why is it on HN?

I didn't get that feel. Codementor had some calls to action, but that's common for content marketing; there's a lot of similar articles here where a developer is selling their ebook or course while providing good value in the article. As for it being an ad for Drift, I didn't get that; it felt similar to something you'd read on Indie Hackers.

I dig the Codementor advertisements after every other paragraph.

I counted 3 (+1 in top caption) in the entire article. Probably less intrusive than typical newsletter popup so common among lean startups (disclosure: I've done work for Codementor in the past)