ISIS's current philosophical standpoint is within the 20 year rule. It is a matter of debate, but Salafism is considered by most that I have seen the most important philosophical influence. Here's an academic writing against that position, but mostly from the stance that "we don't have great data", not that they are something else (other than "mixed").

Shirk is a very old Islamic concept. It is condemned universally from our earliest sources. What does and doesn't count as shirk, however, has been a matter of debate throughout Muslim history. Ottoman and Persian miniatures, for instance, are comsidered shirk by some, the pinacle of those empires' art by others. Salafism certainly does not have a monopoly on calling generally accepted items "shirk" (there's some literature to suggest that "religous entrepreneurs" of all stripes can make a name for themselves by taking particularly rigid stances like condemning widely accepted things).

Iby Taimiyya is often seen as the predecessor to many Salafi practices we see today. He was part of the Hanbali school but was controversial in his day because many of his rulings he said were not just binding in Hanbali jurisprudence, but to all Muslims. He did make at least one notable decision on shirk, but is most famous for his decision on takfir (declaring Muslims to be non-Muslims--this was particularly innovative), dealings with Muslim governments who use "man-made law" (not Sharia), the obligatory nature of violent jihad, and the punishment for insulting Islam/the Prophet Muhammed (a Christian was found to have insulted Islam, and then committed penance and converted; most scholars held this nullified the death penalty, but ibn Taimiyya felt very strongly that, universally [not just for Hanbali jurists] the death penalty still held).