Here are the key excerpts on Sundaland from Wikipedia:
"Previously, humans were believed to have migrated southward, from the East Asia mainland to Taiwan and then to the rest of Maritime Southeast Asia...
...A 2009 genetic study published by the 2009 Human Genome Organization Pan-Asian SNP Consortium found that Asia was originally settled by humans via a single southern route. The migration came from Africa via India, into Southeast Asia and what are now islands in the Pacific, and then later up to the eastern and northern Asian mainland.
Genetic similarities were found between populations throughout Asia and an increase in genetic diversity from northern to southern latitudes. Although the Chinese population is very large, it has less variation than the smaller number of individuals living in Southeast Asia, because the Chinese expansion occurred very recently, following the development of rice agriculture - within only the last 10,000 years. (underscoring mine)
Oppenheimer locates the origin of the Austronesians in Sundaland and its upper regions. Genetic research reported in 2008 indicates that the islands which are the remnants of Sundaland were likely populated as early as 50,000 years ago, contrary to a previous hypothesis (Bellwood and Dizon, 2005) that they were populated as late as 10,000 years ago from Taiwan...
Botanists often include Sundaland, the adjacent Philippines, Wallacea and New Guinea in a single Floristic province of Malesia, based on similarities in their flora, which is predominantly of Asian origin..."
I guess this throws the Out-of-Taiwan Migration Theory... out of the window.