The issue is that the inert nitrogen has a really important role in carrying away heat from the flame. Fire is a dynamic reaction where temperature and speed is determined basically by how much heat a set of reagents produce/how much heat is carried away in the waste products. Nitrogen doesn't participate in the chemical reaction, but it gets heated alongside the waste gases, and so it reduces the temperature of the fire.

In a pure 0.2bar oxygen atmosphere, anything that would smoulder or burn slowly in standard atmosphere burn rapidly with a bright flame instead, and some things that you'd normally not consider flammable can sustain flame.

This is the reason the space station maintains a normal atmosphere instead of a thin pure oxygen one.

Conversely, if you double the amount of nitrogen (without reducing the amount of oxygen) in the air, almost nothing burns anymore.