He does make one key assumption wrong: investing is a zero-sum game. That is, if you can't be the smartest guy in the room, you'll lose. Average is perfectly O.K. Average returns, combined with above-average savings produces vast sums of money.
Actually, I think the rest of his advice is pretty good overall.
1) Pay off debt
2) Figure out your budget
3) Buy your necessities in bulk
For people who are just starting out, these kinds of things are great. Also, his own personal investing is more of an active style - that is, spending a lot of time doing research before diving in. This is a sound method of investment, but not reasonable for us who work full-time jobs. Mark Cuban can sit down and study a trade for weeks at a time. He's so passionate when he talks about it, it's clearly a hobby for him. I don't have time for this, so I go the index fund route. Probably 95% of everyone else on this forum should too.