Lal wrote something about it here:
There he wrote:
“There are four “dīpās” inhabited by humans centered on the Earth (in bold in the above paragraph). Jambudīpa is just one and it includes all the countries, not only India and Sri Lanka.
The other three “dīpās”, we cannot see. They exist around the Earth, but are invisible to us, just like deva and brahma realms are also invisible.”