Firstly, the word "Min" here is used for Tab'ieed, meaning this knowledge of the unseen is only a fraction of his total knowledge. Also, it is a general statement which means, according to him, RasoolAllah has total knowledge of these things. For it to mean partial knowledge, the author would have mentioned it. Secondly, is what is recorded on Al-Lawh al-Mahfooth not considered to be from the Ghayb? RasoolAllah doesnt posess knowledge of the ghayb (partial or complete). This is mentioned in the Quran and in the authentic sunnah. Here's example : وَلَوْ كُنْتُ أَعْلَمُ الْغَيْبَ لَاسْتَكْثَرْتُ مِنَ الْخَيْرِ وَمَا مَسَّنِيَ السُّوءُ