This quote tells us a few things about history and the nature of historical evidence.
First, it tells us that it is not always easy to know which historical witnesses are telling the truth. It is clear that not every account written of a journey to some far off land (or account of things that happened in some past time) is true. Occasionally, however, they are. It is hard to know which is which unless you are able to actually go to the place involved and examine the evidence.
Second, it tells us that historians are prone to interpreting evidence through the lenses of their own prejudices and beliefs. Marco Polo's account seemed outlandish compared to what historians once believed to be true. Because the evidence was contrary to their view of what was likely to be true, they discounted the evidence.
This quote shows some of the difficulties of studying history -- we often have to rely on accounts that may or may not be completely truthful. In deciding what is true, we are handicapped by our own preconceived notions about what should be true.