I agree. Although you can debate that Sikhism was informed by Hinduism and Islam or it is synthesis of both (the percentage argument is subjective) it does not detract from the fact that Sikhs are Sikhs. They are a organic people and have unique identity as any othew religion.
The dharmic this and that is just plain "grab" by Hindus like they normally do with most things within the borders of British Raj. Judaism, Christianity and Islam share some aspects but noboy says they are same.
And the fact that they went the India had more do with politics and power play rather than their feeling buddies with Hindus. That also could be applied to Parsi and Christians who mostly went with India.