Did he really say that?

"Practical men, who believe themselves to be quite exempt
from any intellectual influences, are usually the slaves of some
defunct economist."
-John Maynard Keynes

Apparently he did.  Consulting the Oracle Google, the following was found at Wikiquote, as an excerpt from Keynes's famous tome, The General Theory of Employment, Interest and Money:   The ideas of economists and political philosophers, both when they are right and when they are wrong, are more powerful than is commonly understood. Indeed the world is ruled by little else. Practical men, who believe themselves to be quite exempt from any intellectual influence, are usually the slaves of some defunct economist. Madmen in authority, who hear voices in the air, are distilling their frenzy from some academic scribbler of a few years back. I am sure that the power of vested interests is vastly exaggerated compared with the gradual encroachment of ideas.

• Ch. 24 "Concluding Notes" p. 383