When do I add the chicken stock in this recipe? The author doesn't specify
After the wine.

Let the wine almost reduce, then add the stock. Follow the recipe then from "reduce by two thirds", reducing the now wine-flavored stock.

You might as well be quoting any of 1000 recipes for mushroom risotto. It's basically a typo, the author has seen it so many times before that he doesn't see it.

I can't know exactly the author's intentions, but I'd bet my bottom dollar that those are pretty close.