This follows from John's first claim. John states that a great writer does not need diversity in subject matter. So even if Favilla's subject matter isn't particularly varied, that fact alone cannot be used to exclude her from being considered a great writer.

This contradicts John's second claim. John says a great writer must have the ability to explore a particular theme deeply. If Favilla can't explore any theme deeply, she fails to meet John's necessary condition, and John would say she doesn't deserve to be considered a great writer.

John says the ability to explore a theme deeply is necessary for being a great writer, but he never says it's sufficient. Even if Favilla can explore a theme deeply, John might require additional qualities before declaring someone a great writer.

John says varied subject matter isn't required for greatness, but that doesn't mean having varied subject matter prevents greatness. John's claim is that lack of variety can't disqualify you; he never suggests that having variety can disqualify you. There's nothing in John's statements that would commit him to saying Favilla wouldn't deserve greatness if her subject matter were varied.

John never mentions distinctive style. Although Muriel brings up Favilla's distinctive style, John's own criteria involve subject matter diversity (which he rejects as a requirement) and exploring themes deeply (which he endorses as a requirement). Having a distinctive style is simply not part of John's framework, so he isn't committed to any position about whether lacking one should disqualify a writer from greatness.