While it is true, millions of Native Americans lived in the "New World", and others, including Vikings had reached America from Europe previously. But Americans and Europeans tend to be Eurocentric, and Europe was where the power was and it took old Chris to find the Americas and take the news back to Spain and to the leaders of Europe. The colonization of the Americas, bad for the Natives, started with Columbus, not the Vikings or some group that has legends of finding America before Columbus.
I don't know if the Vikings told anybody in Europe or not. I think their discovery was not known for a long time, maybe until the 20th century when artifacts were found in Newfoundland. Before that it was legend.