If I remember correctly, the entirety of that quote is really more about being a well-rounded person than about the best way to make a living. I'm betting Robert Heinlein didn't make his own clothes and shoes, though maybe he could have.
Yeah, even in a world where everyone can, to complete the quote, “change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly”—there will still be some who are particularly excellent generals, butchers, sailors, architects, poets, accountants, stonemasons, medics, nurses, soldiers, officers, engineers, programmers, and chefs; they will simply also be capable of doing a passable (if perhaps mediocre) job at the other tasks as needed. And frankly, merely being that well rounded by itself takes you beyond mediocrity.