Yes, Cooper probably did say or write something close to that. He also wrote this:Wasn't it Jeff Cooper who said "If you can't solve the problem with six shots you probably can't solve the problem". Or something like that.
From Cooper on Handguns by Jeff Cooper, 1974:
"The Peacemaker was most definitely a service pistol in its day, but that day is past. Now it is purely a sporting weapon -- a plaything. Not that, in proper calibers, it cannot serve to stop an argument as well as it ever did, but it is no longer the first choice for that task. One can still sail across the Atlantic, but that is not what we do if merely crossing the Atlantic is our object. Likewise, we can still wear a Peacemaker for personal protection, but it is not a particularly good idea...I actually carried it in the early days of the Pacific action (on the advice of a still-active gun writer whose honesty I have reason to question) until I found out the hard way that the U.S. ordnance department knew more about the subject of fighting handguns than many people." (p. 157)
And:
".. I cannot deny that I am no great admirer of the Peacemaker, and this is not simply because I was frightened by one (my own) when I was a very junior officer. Trying to reload that relic quickly, in the dark, in the rain, in the mud, was a task I grew very tired of in one quick session." (p.158).