What people have to remember about Islam is that it is a non-hierarchical religion. (Especially true of Sunni Islam.) There is no pope, no cardinals, bishops, etc. Any imam can issue a "fatwa" (religious declaration). However, the authority and power of that fatwa is only determined by however many Muslims respond to it by crying "Allah u Akbar!" and following the guidance it spells out. Otherwise it's meaningless. And that quote from the Marrakesh Declaration shows that there's no shortage of Muslim scholars who don't agree with what ISIS and AQ are preaching.
So who's to rule on whether the Taliban is right or the Saudis and the Iranians are right on the subject? Answer: Whoever has the power to enforce the rules.
In case you hadn't noticed that is exactly like the Xtians today. Any meathead can start a "church" and promote any version of the gospel they feel like. And there are hundreds of them some even with enviable TV shows and incomes.
have another day
Dr.WtS