@andrewgthomas I'm in school right now as well, and I have only ever been encouraged to freelance (though I haven't had much time or opportunity to personally do so). What I've always been told by professors and recruiters is that personal projects and freelance work are always better in portfolios. They can usually tell when a project is school work because it doesn't always seem as genuine as a project you'd have done for yourself or as sophisticated as professional work. (My opinion is that personal projects are really like freelancing for yourself.) Gaining professional experience whilst in school can only help you understand your industry and begin navigating it so you aren't caught unawares once you graduate.
From what you said, it seems to me that maybe Chris Do discourages students from freelancing whilst in school because they still need to learn a lot and he doesn't want their attention to be divided. But in reality, we never stop learning and once someone is willing to pay you for your services, you're professional whether or not you are still in school. School work is a great learning tool, but generic prompts rarely grow a professional, sincere, or interesting body of work that is attractive to consumers. What does he suggest recent grads put in their portfolios? Just schoolwork with the same assignments all your classmates have as well? And having actual industry experience can easily trump classroom rhetoric.
The point of school is to train you and build the skills necessary to work. Nobody in the industry cares whether you are in school or not. They care if you can do the job. Some of my peers successfully freelance and do their schoolwork. Also the more experience you have, the more equipped you are to handle new things that come your way. It's really up to the individual student and whatever stage they are in their art journey, but I can't understand discouraging an illustrator or commercial artist from developing their business and gaining an income as soon as possible.