The last sentence in the same verse Al-Kahf 18:105 ⧉: فَلَا نُقِيمُ لَهُمْ يَوْمَ الْقِيَامَةِ وَزْنًا (and We shall not assign to them any weight) means that their deeds may appear to be great outwardly, but when placed on the balance of reckoning, they will carry no weight. The reason is that these deeds will be of no use and will not carry any weight because of kufr (disbelief) and shirk (ascribing of partners to Allah).
According to a Hadith of Sayyidna Abu Hurairah ؓ appearing in the Sahih of al-Bukhari and Muslim, the Holy Prophet ﷺ said: On the day of Qiyamah, there will come a tall and heavy man who, in the sight of Allah, will not be worth the weight of a mosquito.' Then he said: 'If you wish to verify it, recite this verse Al-Kahf 18:105 ⧉ of the Qur'an: فَلَا نُقِيمُ لَهُمْ يَوْمَ الْقِيَامَةِ وَزْنًا (and We shall not assign to them any weight).'
Sayyidna Abu Said al-Khudri says: ' (on the day of Qiyamah) deeds as big as the mountains of Tihamah will be brought in. But, they will carry no weight on the balance of justice.' (Qurtubi)