OK, so what if the chinese made it to North America before Columbus. What did they do with that information?
Nothing.
Cannot find it now, but there was an article that described some Chinese explorer who discovered America in something like pre-1000 A.D. (not the 1421 book) He described a voyage south along the coast, mentioning mountains that corresponded to those in Washington and Oregon. He later described going inland and discovering "a great rift in the land that went on for miles" that has been interpreted at the Grand Canyon. Cool, IF true.
No follow-up expedition was made, and years later, the memory of that trip was forgotten and a later emperor declared the report a fable and destroyed it.