Actually I misread: the winders cut their own thumbs off to get out of their crippling obligations. The passage in William Bolts's 1772 book is quoted in later works.

A year later, The Critical Review: Or, Annals of Literature - Google Books quotes the passage with the claim on chopped-off thumbs at length, predicting in a comment that such practices will lead to the decline of "the trade of the East India Company".

More importantly, the first Indian source on the thumbs story I found is another quote of the William Bolts book in the 1902 book The Economic History of India by Romesh Chunder Dutt (page 27, pdf page 25). Here a more complex narrative is set: the combination of the enforced monopoly with extreme profit-taking by the middle-men and tariffs first suppressed Indian manufacturing, and then the price competition from cheaper machine-produced imports was the death knell of India's hand-woven industry. The same passage and the same narrative is repeated in Economic History of India: 1857-1956, edited by Viv Bahadour Singh and first published in 1965.

From what I have seen, all Indian sources with the simplified narrative and the thumb-chopping story located in the early 19th century are later than this, so it is possible that they are an apocryphal misinterpretation of this source.

_{*Lunatic*, n.
One whose delusions are out of fashion.}