I don't know where exactly it states that Islamabad domicile holders are to be treated as Punjab domicile holders, but it is common knowledge that although Islamabad is a federal territory, the candidates from this city are treated the same way as Punjab domicile holders.

I got admission in UET Taxila, which accepts 99% of its students exclusively from Punjab. I have completed that degree. Also in CSS, 50% quota is reserved for candidates from Punjab. Islamabad is treated like any other city in Punjab, like Lahore, Multan or Gujranwala.

In simple words, as far as employment in government organizations and admission to public universities is concerned, there is no difference between a candidate from Lahore and one from Islamabad.