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I'm actually not surprised at all, but I believe the Taliban are not necessarily comparable to ISIS.
The Taliban, for all of its countless faults, have actually governed territory for over two decades. They did harbor Al-Qaeda operatives in the lead up to 9/11, but the two groups are not one in the same. The Taliban also don't have visions of a grand Islamic State in the greater Middle-East. They seem to be quite content staying in Afghanistan and Pakistan.
ISIS, on the other hand, doesn't really seem to have a governing policy other than slaughtering as many Shiites and Westerners as humanly possible. They would rather wage a limitless war than ever becoming a functioning governing body. And they also strive to gain as much land as they can.
The two groups do share one thing in common though--their backwards and outdated vision of governance. Women are second class citizens and gays are put to death.
I actually think this is more of a PR war of words than an actual declaration of war between the two groups. Iran separates the two sides and I seriously don't see ISIS trying to push through Iran in order to get to Taliban operatives in Pakistan and Afghanistan. And even if they did--the Pakistani military with all the American toys we provided them would give them more than a few problems.