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 Belongs to the enoyl-CoA hydratase/isomerase family.