That may have been Billy Beane's premise, but Michael Lewis' premise was to find out how the A's could win on a low budget. The players the A's were able to get were clearly not the ones they would have had had they had the resources of other teams (you really think Billy Beane would have taken Miguel Tejada, David Justice, and Scott Hatteberg over Alex Rodriguez, Barry Bonds, and Jim Thome?), but they provided the most value the A's could get within their payroll constraints.

The book's thesis is that it is possible to win without overspending if you are ahead of the curve at identifying where you can get more value for less money. Sabermetrics are an example that worked, and because it worked, there was a book written using it as the primary example. But the Moneyball principle is that in order to win with limited resources, a team must break with "conventional wisdom". There's nothing that says sabermetrics are the only potential application of the principle.