This page is not what the link you clicked said it was; sorry about that! When links in the fic you're reading don't go to a real-world webpage or to a fake social media post, they send you here instead, where you get out-of-universe author's notes.
I know it's a very common headcanon in this fandom to adapt book!Armand's past as an icon painter to Armand having been a calligrapher, and I do like that a lot! But I personally think this loses a little bit of the 'punch' of the book, when of course Amadeo would not want to paint the face of Isa ibn Maryam (AS)! However, Delhi is the birthplace of devotional singing (qawwali) in Islamic mysticism, with the first Sufi qawwali school established there two centuries before Armand was born, and selective mutism putting this block on something that was very important to Amadeo and brought him comfort feels like a faithful adaptation. I also think it's funny for Louis to have gone for singers both times.