It's already taken into the account. The CO2 enthalpy of formation has C-C bond breaking "baked in", by convention the "standard" form of an element has zero en

It's already taken into the account. The CO2 enthalpy of formation has C-C bond breaking "baked in", by convention the "standard" form of an element has zero enthalpy of formation. For carbon, it's graphite with its C-C bonds. I did neglect the C-H bond enthalpy, but it's close enough to the C-C bond energy to matter too much.