This is a complicated picture but I think there is some truth to what he's saying and what you're saying too. I trade 99% the futures because I call the entire direction. It works for me but I think there probably is a bigger "edge" in stocks but let me back up...
E-minis - Favored by retailers
I think the best game for retail trader is in the e-minis. No PDT rule, plenty of leverage, no stock specific risk and 24 hours markets. My mentor, Gary Smith, started with the e-mini and my other Dr. Steenbarger also traded them. For me, I think its the best thing that suites my personality. I haven't looked at a stock in years except to gauge the index. I see it as a great way to trade. However, I agree that developing an edge in this market is very difficult. It is a great game and probably the best game for the retailer.
Stocks - Favored by props
There are more games to play in stocks for sure. There are pairs or statistical arbitrage and Maverick wrote about the unique edge that is only possible by tape reading for NYSE listed stocks. It is possible to take a fundamental view or play a relative strength with more precision then futures, as well.
Don't get me wrong: I think that trading the S&P is my favorite game. It is the game I enjoy it and would rather trade it then anything else (or a limited risk/reward proxy that tracks it). Sometimes though, I get an idea that isn't possible with the futures only -- trading the ETFs really opens up the possibilities for relative strength plays. So lot good with e-mini and ETF trading.
.... but....
I do wonder about things. I figure a great stock trader with prop backing would have much better return potential then a great spoo trader. But the spoo trader can trade huge size. Plus, a lot of stock traders are gimmick traders who I doubt have really developed an ability to trade, i.e such as rebates.
My opinion is that stocks are best left to proprietary traders. Plus, there is a tendency to always think the grass is greener on the other side. I mean you'll hear traders talk about their great index trading 10 years ago --same goes for the props at stocks..
I wonder also if some of those stock specific edges such as NYSE tape reading are on the way out (or done and gone?) and that is why we've seen some of those traders/props turn to offering education? Also, of course, most props want to make money from commissions which isn't possible if a trader goes to futures.