Mamet was asked about this quote in The Storytellers: Straight Talk from the World’s Most Acclaimed Suspense and Thriller Authors (by Mark Rubenstein) excerpt on google books

MARK: You once said, “There’s no such thing as talent; you just have to work hard enough.” What did you mean?
MAMET: It’s not true. You know, you can practice forever, but you’re not going to throw a fastball like Sandy Koufax did.

Innate ability exists. Luck is real. Hard work doesn’t always pay off.
You play the hand you’re dealt. Now go make stuff.