In the form of periodically-interspersed sponsor posts which are explicitly stated as being such and which are never mixed with actual content. As feed monetization goes, its about as above board as it can get.
Of course, being as above board as possible would still not be good enough for the anti-Marco brigade stomping around this pale imitation of what HN once was. I certainly wasn't here right from the beginning, so no hipster douchbaggery intended by any stretch, but I swear to those who have arrived more recently: You have no idea how good HN used to be compared to this. Sad, truly sad.
What anti-Marco brigade? A heavily downvoted post and me pointing out that you would in fact get ads if you subscribed to his site via a feed reader? Dude has like half his posts hit the top here, HN loves him. A very, very tiny minority don't.