Not true. The 'charring' effect stops fire propagation, look up FAA docs from 787 certification.
I may tell you an old trick from a composites repairman...
If you have to repair a glassfibre aircraft and do not know the exact layup, just cut out a piece, light it, lett it burn off, sort out the charred layers of fabric and identify their grade and orientation.
Believe me, they do burn once you light them. Completely. Al least if they are a piece, and not a large sample which you exposed to fire in a local area in the center only, in that case you are right. And that is what the FAA tests.