Don't you have Xmas cake in the US?

Do you have wedding cake? I.e. a heavy fruit cake which has been soaked in port/sherry/rum/whisky and then decorated with marzipan and royal icing?

Xmas cake is the same thing.

It's part of the English (i.e. British) tradition, and probably spread to the colonies and trading partners. Which would explain it being known in China.

We also have Xmas Pudding, which is a similar fruity better but it's steamed and served with the thickest cream you can find (which in my youth was clotted cream).