Even if Columbus was aware of Chiesa's work (no evidence for it being widely known is given here) or had independently heard of the Norse discoveries, he could have assumed they had reached some part of Asia already speculatively sketched on the maps of the day.

As others have pointed out, what Columbus believed and what he said in order to acquire funding are two different things. At the time, the prospect of finding an unknown continent would have been less motivating than that of finding a new route to an area known as a source of wealth; only in retrospect is the value of the former obvious.