This is true. kafir is a relative term which means you do not believe what I do and vice versa. Muslim is an absolute term which means someone who believes in la ilaha illalah muhammad urasoolulah.
And believes in the finality of Prophet Muhammed(saw) our rasool-ul-nabi. Anybody else who claims otherwise should be handed out harshest punishment of the land othernwise we would be stuck in discussions and the nation can't move forward. Gen Zia was best when it came to ruling a muslim country. He raised the respect of Pakistan in the eyes of the ummah. May he rest in peace.