The subject of the article are internships which pay very well. Big tech internships are usually between $45-50/hr. Some people I know make more money at internships than their entire annual PhD stipend.
YCombinator is a startup incubator, so ostensibly a lot of people here aren't in big tech, and startups do often use unpaid interns. Programmers might not have the full impression of this, because programming internships are usually paid in my experience, even at small startups. But lots of internships in (for example) marketing or administration/HR are unpaid.