Feynman had read both books. He mentioned the prefatory quote from 'Calculus Made Easy' in an interview given to Omni Magazine in 1979:

"... I had a calculus book once that said, 'What one fool can do, another can'..."

I have this in Chapter 9: 'The Smartest Man in the World' in Feynman's book 'The Pleasure of Finding Things Out'.