Earlier quoted context omitted.
Actually, Sai Deepak mentions that 15% of those classified as SC/STs also wear the sacred thread. Of course, we know 'Shudras' have run empires in India and have held high status before the age of colonialism. Unsuprising that the Western press and academia has the nuance of a brick, perhaps because their own history is so grotesquely checkered.
This is extremely misleading and a copout to suggest that even SC/STs wear the thread. Reality is that 99.99% of them don't. I wouldn't trust Sai Deepak with anything, he's a nut job who for some reason sounds eloquent to the upper class Hindus.
edit: asking this question to clarify since the other reply(ies) take this as a given.