I am not sure they even call themselves as Muslims, but are just referred as such. I know several such communities, and there is a common theme. Decades ago, some Sufi saint passed through their village, they were impressed by him and decided to follow him. Using the limited knowledge of Muslims or Islam, they adopted some symbols or identifiers, and were henceforth referred to as Muslims.
In one community that I was familiar with (not in Haryana), they just replaced the idols with pictures of the Arabic inscription of "Allah" and "Muhammad (pbuh) and decided they were Muslim. They used to worship these inscriptions in the same way they previously worshiped idols, like garlanding it, and paying obeisance.