Sulu and Tawi-Tawi as one island?

Yes, it is true indeed! In the Encyclopedia of Earth , it is claimed that during the Pleistocene period, the majority of the present Sulu Archipelago was one island, separated from Basilan-Mindanao to the north and greater Sibutu (and Borneo) to the south by deepwater channels of 205 m and 290 m depths, respectively. The distances between these ice age islands were not great, however.

Further, the precursors of the Sulu Islands were an arc of submarine volcanoes that have existed for at least 25 million years. However, the Sulus were not clearly above-water islands until within the last 15 million years. The islands are low-lying and coralline (limestone). Bongao Peak, on Bongao, reaches 300 meters (m), and Mt. Sibangkok, the highest point on the central ridge that divides Tawitawi, reaches 532 m.

Finally, in the same document, the islands of the Sulu Archipelago were delineated as a separate ecoregion, the Sulu Archipelago Rain Forests, that includes the Tawitawi Group, Tapul Group, Jolo Group, and Samales Group of islands. These islands, with a lowland moist or semi-evergreen moist forest vegetation, are also an EBA and have been identified as a distinct biounit by MacKinnon and a biogeographic zone by the Philippine BAP.