It's not fully charcoal (fully carbonized). Wood is mostly two compounds: cellulose and lignin. They undergo thermal decomposition at different temperatures. By partially burning the exterior, you decompose the cellulose, and when you attempt to set it on fire again, it's harder to light, because you no longer have the cellulose decomposition to provide fuel, but have to get to a higher temperature for the lignin to decompose.

It's charred, but not charcoal. And yes, mostly it just makes the ignition temperature go up. But that's half the battle for fireproofing. You're mostly not trying to stop it from ever burning, but giving people enough time to evacuate, or the firefighters to put it out with water.