It's best to use a combination of both fossils and DNA, because DNA is very useful in getting a distinct picture of the way the genes have travelled, (for instance look at the history of Castanospermum australe in Australia), but can get murky if there has been a lot of travel and mixing, and wild related populations are gone. In this case, fossils provide a useful platform which can be used with the DNA to show the place of origin by tracing gene age with fossil age to gain the travel direction of the domesticated plant.
Anyhow, the species epithet 'Indica' has been applied to many species not originally from India, such as Canna indica, Opuntia ficus-indica and Tamarindus indica, so even if it isn't from India, it doesn't mean the name is incorrect.