The 15th century is pre-history?! My garden wall is older than that (literally).
This is not the oldest known artifact from Eurasia either. It's usually difficult to identify things as specifically from one side of the strait or the other, but we have a few examples (e.g. Chinese bronzes [1]) that are clearer.
These beads happen to be an incredibly distinctive artifact that show up basically everywhere the Eurasian trade network reached. We use similar diagnostic beads for Roman/postroman trade in the north sea.
[1] https://www.smithsonianmag.com/smart-news/bronze-buckle-show...
For example, 1788 is considered the end of prehistory in Australia:
Others (e.g. Peter Schmidt, Kehoe) argue that it's just a archaic term without any meaningful communicative value. Resolutions to this include just swapping "prehistory" for "history", or defining it arbitrarily as something like 5000 BCE.
The main result of all of these is to avoid the idea that some random dude shows up for a week, never to be seen again, and using that as the date when "history" starts. European contact in most of the Americas (or Africa, Australia, etc) took centuries from initial contact to full colonialism and the word "prehistory" carries a lot of negative connotations in English. Moreover, the term "writing" itself excludes a lot of written communication systems because it's typically defined only for systems that can record the complete breadth of verbal speech. A system that only encodes a domain of human communication (e.g. tax records and religious stories like Aztec script) is not "true writing" and hence "prehistoric".
It is quite possible that both parties exchanging beads were in the prehistoric period for that part of the world. Then again, maybe not. One party may have had writing. Even if they didn't, the oral histories should be close enough to the introduction of writing to be considered as reliable as writing.