>I thought they were (purported to be) fundamentalists?
Alright, "real talk" the only branches of Islam that can be considered "fundamentalist" are Saudi Arabia and arguably the Taliban sects practicing Saudi-influenced Salafi Islam. Even the Islamic countries that have been "returning to conservative ideals" like Iran and Turkey are the Muslim equivalent of Neocon Christians LARPing. The cultural issues of those regions didn't come about from Islam. They were there long before Islam, and had a similar fate to a lot of the "outskirt" European countries that were converted to Christianity. When the Imperialists (English, French, Dutch, etc.) bulldozed through the outer Islamic territories during the 1800s there was basically a cultural split between those who saw the fruits of Imperialism and wanted to mimic it (most of your modern "progressive" Muslim countries like Indonesia, Singapore, Egypt, etc.), and those who outright rejected it (Saudi Arabia, Bangladesh, etc.). Up through the 80s both groups tried to maintain a global presence, but the Fundamentalists saw American and Soviet expansion into Muslim countries and decided to go roll back their dogma a few centuries. If America had decided to expand into the Indo-Pacific instead of the Middle East, fundamentalist Islam would not be a problem today. Instead America sided with the fundamentalists.
>How does that related to ISIS
ISIS was a cultural movement that wanted to see a return to secular government with Islamists in charge of morality, not legality. The economic and political policies of ISIS are basically a Muslim variant of American legal and political policy in the 1910s-1930s before WWII, but with that classic goat-fucker twist of lighting people on fire and mass executions. Unlike every other Islamic movement that has ever held power though, ISIS was a meritocracy that accepted Muslims from all walks of life, even if they were not well-versed in the Quran. The reason ISIS was considered such a huge threat was because they were recruiting young people left and right, and it turned out actually having Islam in the armed forces was important because shit like the child-wive rapings, butt-fucking, pedophilia, drugs, and other ills commonly expressed in stories from army personnel flourished in an environment where only lip service was paid to religion. The reason people say ISIS are glowniggers js because the CIA supplied them with weapons either directly or indirectly via those various programs that sold military arms to the Mexican Cartels. Right now the evidence points to both, and I wish I was joking about the US selling guns dirt cheap to the Cartels, who resold them to ISIS for access to raw opiates from mammoth poppies.