The Tharus could not have hid out in the jungle for aeons waiting to be discovered during the malaria eradication campaign of the 1950s. They must have a history of their own.
Henry Ambrose Oldfield, in his book Sketches from Nepal, describes the Tharu of the Nepal Tarai as "a puny, badly developed and miserable-looking race, and probably belonging to the same original stock as the natives of the adjacent Plains of India."
Apart from the extreme cultural bias of this description, the belittling terminology was not borne out even in Oldfield's day, when the robust forest-dwelling Tharus were described by another contemporary book as being "chiefly employed in the difficult and dangerous task of catching wild elephants". And a population group that haddefied mighty malaria itself could hardly have been "badly developed".
As for Thant origins, rather than his perfunctory hypothesis, Oldfield might have delved into the possibility that the Tharu have Mongolian blood, but he probably was not interested.