It's been mentioned that it's below the fold, but it's also worth noting that it's not even on the linked page.
The article page linked to from HN is "http://www.useronboard.com/onboarding-teardowns/", which has zero calls to action on it natively. If I go back to home, yes, I see the button, below the fold, presumably it exists on other pages as well, but it is definitely not on the linked page, which had me questioning why I couldn't find it (and spending an inordinately long time looking for it).
The article page linked to from HN is "http://www.useronboard.com/onboarding-teardowns/", which has zero calls to action on it natively. If I go back to home, yes, I see the button, below the fold, presumably it exists on other pages as well, but it is definitely not on the linked page, which had me questioning why I couldn't find it (and spending an inordinately long time looking for it).