But combustible has a very different implication to flammable, which usually means that the substance would catch fire very easily, rather than just being able to self-sustain burning.

Combustible is another French/Latin word.

That doesn't make much sense. Why would the easily-burnt trust still be intact at this point? It's a really weird comment if it wasn't supposed to mean "already burnt, or unable to burn".

I read it to mean 'already burnt or can't be burnt.' I'm aware of the definitions of 'inflammable,' but it just made the most sense in the context to interpret it that way.

I can't edit it anymore, but I did mean "unburnable". I realize (now) that "inflammable" comes from "inflame"/"enflame", but "in-"/"im-" has lots of use as a negation. "Invisible", "indivisible", "inconceivable", etc.