It's somewhat awkwardly put. I think the distinction is between a 7 main bearing six and a 4 main bearing six, in that the former gets crankshaft whip under control and gives you more potential for high-end horsepower, which you can exploit with an easily added overhead cam or two. However, the general gist is that the American manufacturers went off down a technological blind alley with the big pushrod V8s and got sucker-punched by the fuel crisis, while BMW, Mercedes, and the Japanese managed to crank out as much power from more fuel-efficient overhead cam straight sixes. There's probably a better way to put it. The point is probably moot now that V6s have taken over the market, but American manufacturers are still off down a blind alley with the big OHV gas guzzlers.