If the hundred dollar bill was in an accessible place and the fact of its existence had been transmitted to interested parties worldwide, then yeah, the economist would probably be right.

That day the student was the 100th person to pick it up, realize it's fake, and drop it

In my opinion, a refined analogy would be:

Student: Look, a well known financial expert placed what could potentially be a hundred dollar bill on the ground, other well-known financial experts just leave it there!