The logic in the question is wrong, a Tamil poet mentioned about spherical shape of earth, so Tamil people invented that? The only true assumption you can be sure about from this poem is that Tamil people would have known about this fact 2000 years ago.
As Tamilnadu is a very small region in India there are chances that some others outside tamilnadu in India found this and the poet used the term long after it was invented. There are a lot of languages in India and Tamil is not the oldest language, Sanskrit language is way too older than Tamil. There are poems and stories in that language too which is older than 2000 years mentioned in the question. The question is a mistake without considering these facts. Since Sanskrit was used all over the India, may be some Tamil people could have invented that, but we can not confirm that in anyway.
Before having 2000 years assumption we need to consider the age of Vedas and Puranas in India. Because in Srimad Bhagavada(a holy book of Hindus), there is clear saying about the shape of earth. It is called 'bhugola'. It's just an example, there are many other places where it is mentioned. This is written in sanskrit which is older than Tamil of course. Please keep in mind that these books are not specifically talking about the shape of the earth, it is used like a usual term in some places. They are not claiming anything about who invented things...
Also FYI, please have a look at these facts too which can not be found using the common sense.
Two thousand years before Pythagoras, philosophers in northern India had understood that gravitation held the solar system together, and that therefore the sun, the most massive object, had to be at its center.
Twenty-four centuries before Isaac Newton, the Hindu Rig-Veda asserted that gravitation held the universe together.
The Sanskrit speaking Aryans subscribed to the idea of a spherical earth in an era when the Greeks believed in a flat one. The Indians of the fifth century A.D. calculated the age of the earth as 4.3 billion years; scientists in 19th century England were convinced it was 100 million years. Many questions, still to be asked.
Actaul Reference should have been Vedas.
But for history.se Reference: http://en.wikiquote.org/wiki/Vedic_science and simple google search