Most likely the land mass at the time wasn't all under water, so the distances might not have been so large as they are today.
For instance after the 12/26 Tsunami a great sunken Indonesian city was found, that had once been wiped out by Tsunami and only heard of in legend
until this year.
Certainly the Pheonicians were travelling great distances and mastering navigation in 2500BC.
Perhaps the "migration" was slave colonies and such?
See this map
Its not a hard stretch of the imagination to see the possibility of sea to land to sea to land again travel to breach the big Indonesian-Madagascar
gap.

