Arguing that there was no trade between Europe and East Asia in the early Middle Ages because there are no written records of such trade is like arguing that the yeti, an apelike creature supposedly existing in the Himalayas, does not exist because there have been no scientifically confirmed sightings.
Conclusion
You can't say that trade between Europe and East Asia did not exist in the early Middle Ages just because there are no written records of such trade.
Evidence
The absence of scientifically confirmed sightings cannot prove that the yeti does not exist, which is used as an analogy to argue against the position that the non-existence of written records means there was no trade between Europe and East Asia.
Evaluate
This argument follows a common style where the author presents a rebuttal or comparison to a flawed reasoning pattern. The flaw here is equating absence of evidence with evidence of absence. The conclusion can be seen as an opinion from the author, reflecting that a lack of certain types of evidence (written records) is not sufficient to demonstrate the non-existence of trade in the early Middle Ages, much like the argument made about the yeti.
Goal
Find the best available meaning match for *That trade between Europe and East Asia did not exist in the early Middle Ages cannot be established simply by the absence of a certain sort of evidence that this trade existed*.