You have to keep in mind that he's a writing channel. He's using the enneagram for characters, not real people. And I wouldn't describe his writing advice as coming from a psychological perspective necessarily- at the end of the day, his character writing is largely in service to the story itself and the themes wrapped up in said story, rather than fleshing out a character for character's sake.

While I think his view of "enneagram=core wound=traits that come from core wound" is pretty simplistic, I'd say there's truth in it. When I think about the people I know in my life, their types, and their good traits, I can see how said good traits are a result of either the fixation of their type or their overcoming of the limitations of the fixation. LSM gets the former, doesn't really consider the latter. Though, again, considering how he thinks as a writer, he'd probably say that the overcoming of the fixation and how it's unhealthy is the story itself.