...except he stated in the article that they work 8 hours a day...
How is 9-6 8 hours a day? They work 9 hours a day. As stated in the article.
9-6 could include a lunch hour.
A sibling post to yours says "Treehouse developer here. It's actually four 8s." so that is likely the case.
Working from 9-5 includes lunch as well. The fact that 9-6 includes lunch, the fact that it needs to be mentioned, is ridiculous, you still spend 9 hours in or around the office.
Did you read the article? They work 32 hours a week.