It is true that Malaysia and Indonesia occupy the same island. If you take a look at the map I've attached for you below, you will see that it is actually the truth. Indonesia contains several islands, and as you can see, a part of Malaysia and Brunei share an island with a part of Indonesia near the Java Sea.
On a geologic map, if the contacts between sedimentary rock units form a bull’s-eye pattern of concentric circles, with the oldest unit in the center, the underlying structure is a syncline.