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.
This passage suggests that the traditional understanding of music might be incomplete or inaccurate because it doesn't account for the use of drone sound in ancient Indian music.