Desulphurisation is required to turn the extracted oil into something that can be used in a car. That process requires cobalt. So yeah the dirty cobalt business is directly supported by oil-based cars as well. However, I think catalyst use vs. battery use of cobalt is roughly 1:5 so your point remains valid (it's used more in batteries than in desulphurisation). Unfortunately I cannot find free statistics to support this 1:5 ratio which is from memory, maybe someone else can supply a link.