Not sure sure about the amount, I read about it on the uncle bens subreddit.

Correct. While the mycelium may eventually cope with a high salt content, it doesn't like it. Actually, it's like plants in that regard. The fungi or plant is forced to put more energy into "finding" a better water source. In both cases, fruit development will get stunted.

The general problem is that salts can reduce or reverse osmotic pressure. Instead of the fungi/plant being able to naturally absorb water from its substrate, the water will naturally flow into the saltier medium: It will flow out of the mycelium or roots and back into the soil and likely cause cell damage.

When you fruit fungi, it pulls metric ton of water. A mushroom is nearly 90% water, after all. It will suck every drop of water out of your substrate block if you let it. Creating an environment with as little stress for it to pump as much water as it can is ideal.

Edit: Also, I posted here with a picture of mushrooms growing right out of a bag. While the rice does provide nutrients for growing, another one of its functions is to be a water source. You can get a decent flush from just a bag of rice by adding a simple top layer of coco coir and misting it so the fruit doesn't dry out.