Fresh straw holds sugars that hundreds of fungi and bacteria can eat. Spawn it directly and moulds outgrow the mushroom in days. Composting changes that: heat-loving microbes consume the easy sugars, turn ammonia into their own protein, and leave a dark crumbly material rich in microbial bodies.
The button mushroom digests that material. Most competitors cannot, or not fast enough. That is what ICAR-DMR means by a selective substrate. A formulation only sets the process up. Selectivity is created by the process and judged by measurements at the end, not by the ingredients you started with.