Artifacts from the Old World could wash up naturally on the shores of the New World, it does not necessarily mean that some one from the Old World brought them. After the big tsunami in Japan, items washed away by the tsunami washed up on the western shores of North America, these items were not brought by people, for example. The Viking artifacts aren't isolated artifacts, like an iron fish hook, but part of an entire settlement that was found.
When we say "who discovered America", we really are saying "who discovered America for the peoples of the Old World"k it is understood that obviously the natives had discovered it first. But unlike Columbus, the Native Americans could not tell you where America was in relation to other lands, Columbus could. Before Columbus, America and the Old World might as well been on other worlds for all the interaction they had with each other.