I have ground up spent mushroom blocks to make soil
in my backyard, for the veggie garden. They were great.
I think that that is what people are referring to when they
refer to mushroom compost, but when making a soil that
i want more confidence in, then I would use a special fungi
mix purchased for this purpose. (think "Great White" though
there are cheaper alternatives)
Good luck,
JD
P.S. I think that a good question would be to confirm the fungi
species that good for soil in general, and that make mushrooms.
(i.e. is Pleurotis enough? Or Shiitake?)