You are exactly right. I read somewhere most of the students in the MITx classes are working professionals. The attrition rate is extremely high as well: 120,000 enrolled and 10,000 left @ mid-term exam.
Not to say MIT couldn;t produce an introductory course “for the masses” if it wanted to. But it false to represent what they have there now as such.
Looked over their lecture notes for some other CS classes too. Their lecture notes skip over a lot of intermediate steps that 98% of students would need to see. In short, its not written for your anyone who doesn’t already know the subject matter (or is a genius to begin with).
Perhaps they regard real introductory material as beneath their brand. But, .....if I wanted to be conspiratorial, I’d say this was a honeypot to draw prospective students in, totally humiliate them, and then thereby reinforce why they need in-class instruction.