I believe what they really mean is their "Mushroom Broth" rather than Mushroom Stock. See p. 6•19.
Lachyg wins the sleuthing prize! He found the recipe for mushroom stock, on page 5-129, buried in the middle of a recipe for ragout of grains. It calls for 2 kg of mushrooms, 100 g of grapeseed oil, and 800 g of water.
At first I thought, "Wow, that seems like a lot of grapeseed oil!" But on looking up the recipe, I see that they are coating the mushrooms in oil, roasting them at 175C/350F until golden (30 minutes), then simmering in water for an hour, and straining.
From a culinary linguistics standpoint, this makes more sense, because a broth is a finished, highly flavored essence in it's own right, whereas a stock is used in combination with other ingredients to make the finished dish.
However, the differences are interesting. If you scale up the mushroom broth ingredients to the same scale as the mushroom stock, you would have 2kg of peeled(!) mushrooms (Crimini, in this case), 3575 g of water(!!), 121 g of olive oil, and 529 g of shallots. Obviously the shallots would add a lot of flavor, but I'm not sure that it would be a mushroom flavor. The recipes also differ in that the broth is sauteed, vs. roasted, but I doubt that makes much difference in the taste. Likewise, I don't know that pressure cooking for 15 minutes vs. simmering for 30 minutes would make all that much difference.
I thought I would double-check this recipe with Escoffier's Le Guide Culinaire, but surprisingly, I couldn't find any kind of a mushroom stock listed, even under morels or champignons. Cream sauces, and other preparations, such as a mushroom sauce made from Demi-glace, yes, but not a separate stock.
Now I guess I'll have to try both methods.