I think the point was that these are positions that are easier to "hustle" your way into with a minimal investment. ("Read the books above for 30 minutes a day, and you'll be an expert in 90 days!") Compare this to becoming an expert in C - 30 minutes a day for 90 days is not going to give you enough experience to waltz into a startup and demand a job.
(Also, in my mind, the word "hustle" occupies the same bucket as "scam", and "30 minutes a day for 90 days" sounds like a late-night informercial for weight loss.)