It is my opinion that the lower part of the map represents Terra Australis Incognita and  in particular the northern coast of Vietnam. The important gulf on the left  is Tonkin Gulf. Probably  the missing part of the map reached Thailand.

In particular the shape of the map in this part  is a orizontal devepolment of coasts and corresponds with mythical Terra Australis.

Another possible interpretation may bring to Thailand

       Terra Australis  in Piri Reis' map

At the bottom Terra Australis in the map of Lopo Homem (1519)