To what extent are the doubts raised on the aforementioned website valid?

There is no definitive proof of the existence of Uthmanic masaḥif (plural of muṣḥaf) today. There is considerable doubt regarding the authenticity of attributing the available ancient masaḥif to Uthman (may Allah be pleased with him), especially if they contradict the sound, widely transmitted recitations (qira'at) or do not conform to the rules of the Arabic language. Some of the alleged masaḥif contain clear linguistic errors. The non-preservation of the Uthmanic masaḥif does no harm, because the preservation of the Qur'an relies on widely transmitted narration and receiving it from trustworthy individuals, in accordance with His (the Almighty's) saying: "Indeed, it is We who sent down the Dhikr (the Qur’an), and indeed, We will be its guardian."