I'm doing a math PhD and I feel ambivalent about this statement even for myself.

In Devlin's article, doing math is conflated with a formal education in it. He should know better as he has previously endorsed heavy criticism of the latter: http://www.maa.org/devlin/devlin_03_08.html

"...our present system of mathematics education is precisely this kind of nightmare. In fact, if I had to design a mechanism for the express purpose of destroying a child’s natural curiosity and love of pattern-making, I couldn’t possibly do as good a job as is currently being done— I simply wouldn’t have the imagination to come up with the kind of senseless, soul-crushing ideas that constitute contemporary mathematics education." (pg. 2 of Lockhart's article)