The mycelial sheets are rendered 'inert' through one of an option of processes (for example baking it).
A clearer visualisation of the materials here: the mycelial sheets used in research / production are as to spores as a plank of apple tree wood is to apple seeds.
When you're working with the wood you do need to be cautious of the dust, but you don't need to be concerned about apple seeds sporadically appearing.
Reishi on the other hand... eats trees.
For some possibly useful context. When wood (lignan) first evolved there was nothing that could break it down. And for possibly thousands of years much of earth became "polluted" by dead trees that couldn't be broken. It was fungi that finally evolved ways to decompose it. They're obviously really good at it at this point, but hopefully that gives you a sense of just how specialized an organism has to be to primarily live off of wood
You got nothing to worry about :)
I believe the researchers are proposing a fungal substrate for PCBs.
Fungi are like plants. Some are beneficial. Some are not.