From the article…

Since straw bales are tightly packed, they do not burn as easily as certain other materials. The tight packing reduces airflow, something that is critical to sustain a fire. While there is some fire risk during the construction phase (as is the case with many building materials), once the home is finished, it’s flame retardant nature decreases the risk, usually resulting in a home that better resists burning than a traditional stick built house.