Well technically not stateless, because they'll be Canadian once they take the oath... And yes, they will have no legal status in India once they take oath and need to apply for a Visa or leave the country forthwithI didn't realize that an oath in India (which is unlikely to happen anyway) would result in an immediate loss of Indian citizenship and render this person "stateless" and without official status in India.
But yes, the Indian Citizenship Act, 1955 provides for automatic loss of citizenship of one voluntarily takes up another countries citizenship by swearing an oath or by registration or by other means as allowed by the other country