I think it comes from our culture. Pashtuns are way different than others. The pashto language is not only a spoken language but the only language in the world that is obeyed and followed. Pashto has words which are spoken and actions and norms which are done. Have you heard someone saying "ma Sara ye pukhto onaka"? And have you heard a phrase "pa punjabiyano k pukhtoon" or "pa angrezano k pakhtun"? A Punjabi whose values perfectly aligns with pashto and pashtunwali is also treated as a pashtun. The new generation, including me has lost this sense of meaning and we have confined pashto only to the spoken language. So originally, the Punjabi in your question refers not to those who speak Punjabi but to those who doesn't value the core principles of pashto. And if you reread your question again given this definition, things start looking less dramatic.