Is this recipe missing a liquid ingredient?

The exact amount of liquid you end up with will depend on how much is in the mushrooms, so the final product may vary a bit. But you would be surprised at how much water a mushroom actually contains, and the salt they are coated in for the overnight rest will draw out most of it. I make a version of it myself, and it's a wonderful umami bomb to have on hand. You can use it in places where mushrooms are paired, add it to a pan sauce or gravy, marinade, soup- just about any place savory. It is salty, as you might imagine- so you would want to adjust your recipe to allow for that.