It looks legit. There are lots of retreats/workshops like this around the country and the world, though most are so slammed with interest that you have to apply to be accepted into the retreat. I'd guess this is a rather new retreat since there seems to be no application process--you simply pay and you go.
Now, the retreat being legit doesn't necessarily mean it's worth it. Do you know the directors? Are you interested in studying with them? What benefit do you get from this retreat that you couldn't have by taking yourself somewhere for a weekend to write and work on your own?
I don't have answers to those questions, of course, and know nothing about the directors' talent or ability as teachers/workshop leaders.