What a stupid question.
Punjabi is an ethnicity, it has nothing to do with religious identity.
Punjabis can be Muslim, Sikh, Hindu or even Christian (I knew a convert to Islam whose parents were Indian Punjabi Christians).
Not all Sikhs are Punjabi, I've seen some English ones on TV even though Punjabi is probably the language of liturgy for them as Arabic is for Punjabi Muslims and Latin is for Punjabi Catholics.