According to the article the minimum working hours isn't 20 hours a week, they are forcing their employees to block off 20 hours during core working times to be available in case Stitch Fix wants them. If Stitch Fix guaranteed those 20 hours were paid I'd be more understanding.

While the new policy requires that stylists be available at least 20 hours per week, company guidelines reviewed by BuzzFeed News said they can be scheduled for as little as zero hours as “availability does not guarantee a certain number of working hours each week."