"Markets make managers” means that, for some money managers, it pays to be lucky as well as good. If the stock market goes up (or down) as the money manager had predicted, the market has “made” the manager.

A money manager shouldn’t accept too much credit (or blame), as the adage “Don’t confuse brains with a bull market” illustrates.


The Independent (London)
Twilight of the fund gods
By Jonathan Davis
Wednesday, 10 May 2000
(...)
In a world where styles and methods periodically move from failure to success, and back again, it remains the case that, as the investment guru Dean LeBaron puts it: “Markets make managers. Managers don’t make markets.”

Google Books
Index funds:
The 12-step program for active investors
By Mark T. Hebner
Irvine, CA: IFA Publishing
2007
Pg. 206:
Markets Make Managers
There is one other point regarding the futility of attempting to identify skillful money managers. An old investment proverb observes that “markets make managers.” This means that if the market favors a money manager’s particular investment style anyone can achieve outstanding performance.