It indexes the PDFs in the zip files. It's not just the courses, but the full text of all the lectures in each course.
What I meant is that I would prefer to see where exactly my keyword surfaced in the course - in what lecture or assignment vs lists of all courses mentioning the keywords.
Ah, sorry I misunderstood. Yes I completely agree. The "snippets" shown right now are just the official course descriptions. Better snippet generation is something I need to research before I deploy anything. It's a complicated problem to solve in a way that will work across all searches. Google used the meta description tags for years before they figured out how to best show snippets from page contents.