Perhaps the greats like Bharata, Sage Matanga and Shaarnga-deva did not know the secret of tuneful tones (up to acceptable level of normal human ear, on the basis of taanpuraa drone) for they do not mention use of drone sound for any of the musical purposes.
It's possible that renowned musicians like Bharata, Matanga, and Shaarnga-deva were unaware of the technique of creating pleasing tones (within a range suitable for human hearing, based on the taanpuraa drone) because they don't mention using drone sound in their musical compositions.