the rapidly expanding nature of a fire
Yeah, and fire is "un-natural"--at least to human perception--in the way it propagates.
"Our" general experience is with small, controlled things like camp fires, that as slow and reluctant to catch fire, and want careful tending to keep going. And, that's actually on purpose, we want our deliberate fires to be controlled, and to "fail safe" to unlit.
Uncontrolled fires need more than one person to cope with (the fire services use the term "attack" deliberately for how they address fires).
SO, in our Original Premise here, one of the problems a single self defender faces versus a group threat are threat coming from multiple directions. Add in a fire--which will focus a person's attention, as well as overload the senses--and the number of extra "eyes" required multiplies along with the need for additional arms.
While writing all this, another issue occurs to me. One of the major dangers, an invisible one at that, and deadlier than flames--are the combustion products from fire. "Smoke inhalation" kills far, far more than actual flames. So, yet one more risk the home/self defender faces with fire are the combustion products.
Again, nothing leaps to mind as an adequate answer, short of living in the firehouse (and even then, they often leave the house empty to go on calls).