Carbon based expression is misleading as usual. Organic based life (i.e. based on carbon structure chemistry) is more accurate.
Cyanide is carbon based. Kills you quick. Aromatic hydrocarbons like benzene will kill you slower.
It's like the "all natural" misnomer. Coral snake venom is all natural, but I don't want any, thank you.
SO what? Most of the life on earth is still carbon-based, as in with carbons at the centre of all essential molecules in biology. What kills you is irrelevant to make that statement true or false.
Breathing pure oxygen is also harmful.
Or drinking too much water.
The Adam Ruins Everything segment on this was great, it takes a large deal of water to poison yourself but somehow people manage to do it because of myths about hydration.