The problem with the letter is, it is not a clear example of the Koran. No Koran references are given.
This is only about Your Ignorance in Surah of Quran and interpretation of this old arabian text.
The Jihadist activity is in contrary with the Number 21 of this letter.
But You have no clue where is about that in Quran.
That is about An Nisa, 4:59.
I'm sure, if You goes to listen it, in some translation, then You will have any sospect of such hard rule, that goes from it.
The letter is 16 pages long, there is described clearly Why, with all references to text of Quran.
You need better to check the site, for find the full descriptions of any point of summary.The Number 21 have too a full page of text with the reference and clear significate.
That is why this fucking sectants are killing mullah in Northern Caucasus, for them is a easy job to show the difference between sectants and the Islam.
https://jamestown.org/program/muslim-clerics-in-the-north-caucasus-between-the-hammer-and-the-anvil/