In chapter 1. Jared Diamond uses the term "Eurasia" through out the book. why does he insist that Asia and Europe is actually one continent ?

nope

"Because Eurasia is not bisected by any major barriers"

This isn't the exact quote, but close enough.

On a side note, the Ural Mountains actually separate Europe and Asia (major barrier), but pay no mind to me. Just your grade.

In the sources is an audiobook on Youtube. You'll find the answer at 36:38.