yes true that is not related actually...but GMAT does this this to confuse the test takers...by merging two things...
first the topic is regarding historians finding reason that how the ivory artfact number increased...then suddenly new topic is merged(which is rather main topic for this question).===>that historians assumed that trade started during this time....so finally according to question stem you have just to focus on trade thing.
so a takeaway will be : when there is EXCEPT question ...dont prethink the answer...rather understand the argument and then start eliminating wrong answers.
hope it helps
DO NOT PREPHRASING, i will keep that in mind
so one thing just for sure, the author included the ivory part just to introduce the trade thing,
"trade thing" is "the historian's account" ( in the question stem), right?