Ibn Taymiyyah's argument is that the matters recorded in the Preserved Tablet (Al-Lawh Al-Mahfuz) are known only to Allah and no one else.
Ibn Taymiyyah quotes:

وليس لأحد اطلاع على اللوح سوى الله.

وما يوجد في كلام بعض الشيوخ والمتكلمين من الاطلاع عليه فمبني على ما اعتقدوه من أن اللوح هو العقل الفعال، وأن نفوس البشر تتصل به، كما يذكر ذلك أصحاب رسائل إخوان الصفا .المستدرك على مجموع الفتاوى ١/١٣٨

"No one has access to the Preserved Tablet except Allah. What is found in the statements of some scholars and theologians regarding having access to it is based on their belief that the Preserved Tablet refers to the active intellect (Al-'Aql Al-Fa''al), and that human souls can connect with it, as mentioned by the authors of the Epistles of the Brethren of Purity (Ikhwan al-Safa)."
(Al-Mustadrak 'ala Majmu' al-Fatawa, 1/138)

However, in the year 702 AH, during the battle against the Tatars, Ibn Taymiyyah predicted the victory of the Muslim army and swore to it more than seventy times. When people repeatedly questioned him about this, he responded by saying that Allah had decreed in the Preserved Tablet that the Tatars would be defeated this time. Ibn Al-Qayyim records this in his book "Madarij al-Salikin":

ثمّ أخبر النّاسَ والأمراءَ سنة اثنتين وسبعمائةٍ لمّا تحرّك التّتار وقصدوا الشّام: أنّ الدّائرة عليهم والهزيمة، والظّفر والنّصر للمسلمين. وأقسم على ذلك أكثر من سبعين يمينًا. فيقال له: قل إن شاء الله. فيقول: إن شاء الله تحقيقًا لا تعليقًا . سمعته يقول ذلك. قال: فلمّا أكثروا عليَّ قلت: لا تُكثروا، كتب الله تعالى في اللّوح المحفوظ أنّهم مهزومون في هذه الكرّة، وأنّ النّصر لجيوش الإسلام. قال: وأطعمتُ بعضَ الأمراء والعسكر حلاوةَ النّصر قبل خروجهم إلى لقاء العدوِّ  (مدارج السالكين لابن قيم ٣/٣٠٦)

Then, in the year 702 AH, when the Tatars mobilized and headed towards Syria, he informed the people and the leaders that the defeat and loss would be upon the Tatars, and that victory and triumph would be for the Muslims. He swore to this more than seventy times. When he was told, "Say, 'If Allah wills,'" he replied, "If Allah wills, as a confirmation, not as a condition." I heard him say that. He said, "When they pressed me further, I said, 'Do not insist. Allah has decreed in the Preserved Tablet that they will be defeated in this battle, and that victory will be for the armies of Islam.'" He added, "I even served some of the leaders and soldiers the sweetness of victory before they went out to face the enemy." (Madarij al-Salikin by Ibn Qayyim, 3/306)

Accusing the Sufis of believing that Allah grants His awliya (saints) knowledge of what is written in the Lawh al-Mahfuz, dismissing it as a false philosophical idea—while at the same time asserting knowledge of the unseen (ghayb) and claiming that the Mongols' defeat was decreed in the Lawh—is a clear contradiction. How can one reject others for holding such beliefs while making a similar claim oneself?