Why should a Punjabi who has always lived in current day Pakistan, who's ancestors never lived in any part of current day India claim to have Indian identity... ? They can however claim to belong to Indus Valley civilization.
What the historians or ancient empires attributed to India in the past is now divided among various countries in the Indian subcontinent.
Further more, Punjabis and Pathans of current day Pakistan have had more connections extending westward, than eastward. Our language, heritage, culture, and migration routes have always had much stronger connections to west and Central Asia than to south East Asia/ India. Ofcourse Punjabis have also had influences from people who lived in lands that form part of India now. Just like modern day Indians had from people living in lands part of modern Pakistan.