The majority of Jamacians are of African descent
so they can be both Jamacian and African
since they're the majority, it is safe to assume that a Jamacian is of African descent
I think this is an inappropriate automatic assumption. The vast majority of China's population consists of Han people (91%), however there are also several minority groups. The odds are that you'll come across a Chinese individual of Han origin, but there is also a chance of meeting a Chinese individual who is not of Han origin.
While Jamaica consists of those primarily of African decent, there are also many who are of Lebanese, Sephardic, Chinese, and British decent.