Nowhere in Zhang He's explorer journals does he mention sailing the Pacific or Atlantic off the Americas. His furthest expedition made it to Mombasa, Kenya.
Admiral He divided his fleet of some 800 great shiresps into 5 smaller fleets and sent them in different directions. He was only with one of those 5 fleets so did not travel to the other 4 locations.
So far as Africans settlong Cemtral America, my guess is that some Africans served as mercenaries on some Mediterranian ships (Sea People, Phonecians, Early Greeks). My guess is that ship or ships were wrecked along the Mexican or Central American shores, and then a powerful African ended up controlling the group and then the local population. I believe these giant heads were carved around the same time Egypt was carving giant statues of the late 18th or early 19th dynasties. So an African mercenary could have seen them and wanted to be remembered the same way.