if my understanding of it is correct, myco isn't something that will propagate on its own, without a root host, also it's not going to "grow" much after it does, meaning it's not transient, so where you apply it, is where it stays (for the most part)
and myco is CRUCIAL to the uptake and usage of phosphorus in particular, up to 75-80% of the overall plants uptake of phosphorus.
so a compost or wormbin will be LOADED with micros but just not myco