Answer
(Fatwa: 873/875/N=1433)
The words of Ghair Muqallid mentioned in the question are wrong. It is proved by various hadith that the holy Prophet Muhammad (صلى الله عليه وسلم) offered twenty rakah tarawih. And Hafiz Hajar al-Asqalani says that the scholars of Hadith agree about the hadith of 20 rakah tarawih that it is sahih (authentic).
متفق على صحته :
(التلخيص الحبير في تخريج أحاديث الرافعى الكبير 1:119)
While no authentic hadith exists in the collection of Hadith regarding tarawih being eight rakah. (For detail: See Fatawa Rahimiya Jadid Saeedi 3:246-262)
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband