[PIF]

“... all that these early invaders needed to do was follow the coastline in boats/rafts first going north along the east coast of Asia ...”

Or they could have simply followed the currents to S.A. or N.A. as the Jomon in present-day Japan did 20,000 years ago. They likely used the same techniques as Polynesians used to navigate the Pacific - follow the currents.

[Pharmboy]

True enough...but, it would be easier to obtain food and water (going back to land to hunt and build a camp and a fire) and much less dangerous (storms) than blue water sailing, currents or not.

But, I am certainly not dismissing what you are saying or the evidence for same, just making the case for incremental shore-hugging exploration.