It's always a pleasure to read your work. I think you can take Dillard's comment at face value. She's telling the questioner that if he doesn't like sentences, he should not be a writer. Because writing is all about sentences.
Sentences are what a writer paints with. They are how you create and evoke the world. Especially if by "writing" one is talking about art. Or at least some effort to approach it.
A writer makes a good thing and possibly a lasting thing by making good sentences one at a time. The same way one makes a good house. Writing a good sentence requires one to have clear thoughts, to be patient, and to learn how a sentence can connect in a true and meaningful way with another human being.
Anyone who doesn't like sentences does not really want to be a writer. He's just looking for a lofty way to identify himself. There are lots of people like that. They want to be called writers without any idea of what it takes to become one.
As you can probably tell, Dillard's response made a big impression on me. Hope I didn't go on too long about it.