Ibn Umar on the Quran

In the book "Fadhael al-Quran" written by Abū ʻUbayd al-Qāsim ibn Sallām (a scholar from the Salaf), Ibn Umar says: > Verily among you people one would say that he has found the Quran whilst he is > unaware of what the total quantity of the Quran was, **because most of > the Quran has been lost rather one should say that verily he has found > the Quran that has appeared.**’ > > ~ Fadhael al-Quran - Volume 2, Page 135 I know, this book is not that much popular but the authenticity in Islam is not measured by book's popularity but Isnad & the Isnad of the above quote is 100% authentic as the chain of narration is: > Ibn Umar -> Nafi -> Ayub -> Ismael bin Ibrahim. All these narrators are reliable according to Ibn Hajar and al-Dhahabi. - See **Al-Kashif, vol. 1 page 242 by Dhahabi** and **Taqrib al-Tahdib, vol. 1 page 90 by Ibn Hajar** for Ismael Bin Ibrahim's reliability. - See **Siyar alam alnubala, vol. 6 page 15 by Dhahabi** and **Taqrib al-Tahdib, vol 1 page 116 by Ibn Hajar** for Ayub's reliablity - See **Al-Kashif, vol. 2 page 315 by Dhahabi** and **Taqrib al-Tahdib, vol. 2 page 239 by Ibn Hajar** for Nafi. So: --- How do we explain the saying of Ibn Umar?