is an afterthought = is not the first thought = is not the initial focus = is not the main direction = will be a lower priority

"for others China is an afterthought" For others, China is not thought of as a market in their initial considerations. Only later do they give a thought to, or consider China as a market. There's not enough text provided to work out who 'others' is referring to. You would need to add the preceding paragraph. I assume 'others' is referring to all industries other than technology, automobile and consumer-products.