My first guess was that the reflective surface would act as a thermal barrier against ambient temperature. If that's just the outer layer it may have some actual insulation built-in too. Considering the use-case it'd be reasonable to engineer for an increased tolerance to some hazard whose benefit compounds alongside other precaution and prep.
i.e. the protective material's melt point only has to be a bit higher than the shielded material's flash point for the product to be effective as-advertised. It won't save the cabin if ambient temp hits ignition or combustion temp but the cost:benefit may still make sense.
Searching for "aluminized structure wrap" provides some results which seem to confirm this.
What was the context of your discussion during firefighting training? Accurate to the marketing copy? Less situationally useful than advertised but still better than nothing? Completely worthless?