[HUE]

Most textbook writers by the big publishers (Oxford, MacMillan, Pearson) don't have PhDs. They are (or were) teachers in the field, who then tested the material written again and again. At most universities, the professors choose the textbooks for the class, too.

Here's a job ad with TESOL Journal for someone to put together curriculum, and it doesn't require a PhD: Job Opening: Instructional Designer at McGraw-Hill [TESOL Career Center]

That said, I would hope that the curriculum designers have a good understanding of pedagogy.