The recipe uses the broth you cooked the chicken/mushrooms in to cook the noodles too, but depending on the noodles, they’ll absorb a lot of it during cooking.

So, if you want your noodles etc submerged in broth in your bowl, either make more than a litre of stock (maybe use a little more leek/carrot/ginger). Alternatively, just boil the noodles separately in water, it’s not as if the broth flavour won’t dominate anyway :)