GMAT Critical Reasoning (CR) | Archaeologists in Michigan have excavated a Native American

Still don't understand why C is flawed. As for me the reasoning is the following:

1) Traders were active in the region starting with late 1620'. Therefore, since no goods were found in the camp, the camp must have been held before that period. We need to find something, that strengthens this point by either providing the additional evidence or by eliminating alternative reasons, why the goods were not found.

Option C states that - these goods were especially valuable and therefore there was