Most notably, if you choose just one persona, and that persona doesn’t adequately reflect the diversity of your users’ behavior, or you don’t use the persona to faithfully predict users’ behavior, you won’t find valid design flaws. You could spend an hour or two conducting a walkthrough, and end up either with problems that aren’t real problems, or overlooking serious issues that you believed weren’t problems.
I agree with this. I'm taking 380 as well, and we just did a lesson on User Stories and Personas. This is almost sounds like the One User Fallacy, where a product is designed with one "generic user" in mind, which doesn't exist. This leads to people having their needs missed or creating a solution to a problem that doesn't exist.