What a crock of crap.
"Don't be ambitious: be mediocre and replaceable."
He's right. Don't try to be a youtube star or a professional athlete, or an actor. Do something realistic.
But I mean... do something ambitious for god's sake! Be an accountant! Be a doctor! Be a computer programmer! Hell: Be a lawyer. These are not crazy pipe dreams.
Lets say someone makes $120k as a programmer a year taking out NO debt, and having NO risk invested in the company they work for. That is a damn healthy small business.