In the history of capitalism, this is the hardest time ever to invest.
Admittedly I haven't been around investing that long, but from history books, I could pick out a couple times that have been harder.
With that said, pretty much everything else I think is spot-on. Either you make money with buy-and-hold of value stocks (or generally just the entire market), or with a lot of infrastructure and know-how and huge cash reserves for HFT or similar, or through cheating.
I definitely don't agree with a max of 30% into stocks, because my outlook is probably a lot longer than his, and I'm not picking.
Cash is king is a good idea for people who need cash often. It's a good idea if you're going to start a business. Makes sense. People here would tell you the same if you wanted to, say, buy a rental property. It's not a good idea if it sits but he's not advocating having it sit.
Oh, and I think it's probably easier to save money than to earn more, but maybe that's because he's a wall street guy who sells books and runs hedge funds.