I would take that chart (and its source) with a gigantic mountain of salt, certainly for the pre-modern period. The raw statistics (available here in the 'Historical Statistics' section) have no sources indicated, which leads me to believe that the pre-modern statistics are nothing more than guesstimates, at best. There is simply no way at all to quantify pre-industrial production simply because we don't possess the data, and comparing different parts of the world through just-shy-of-made-up GDP figures is a waste of time.

I'm sure pre-modern India was a very large economy, because it was a very large and populous place. But whether it was the largest in the world is something we'll never know. It would be possible to compare the complexity of various regional and supra-regional Indian economies with other major pre-modern economies (such as China or the Mediterranean), which could actually tell us something interesting, but as far as I'm aware that sort of work hasn't been done, and so the question is many years away from being answered.