Decarboxylates ethylmalonyl-CoA, a potentially toxic metabolite, to form butyryl-CoA, suggesting it might be involved in metabolite proofreading. Also has methylmalonyl-CoA decarboxylase activity at lower level. (2S)-ethylmalonyl-CoA + H(+) = butanoyl-CoA + CO2 (S)-methylmalonyl-CoA + H(+) = CO2 + propanoyl-CoA kcat is 10 sec(-1) for (S)-ethylmalonyl-CoA as substrate (in presence of 5 mM ATP). kcat is 1.68 sec(-1) for (S)-methylmalonyl-CoA as substrate (in presence of 5 mM ATP). Belongs to the enoyl-CoA hydratase/isomerase family.