People have to be very careful when reading these articles. The headline and first paragraph say ISIS claimed responsibility:
However, they specifically imply throughout the article that ISIS did not claim responsibility. (The New York Times and many papers did this with Hamas during the 2014 Gaza massacre. The headlines would say Hamas claimed responsibility while the rest of the article implies that they didn’t.)
ISIS celebrated the attacks. They never said they did it. Here is a translation of one of the messages from the groups encrypted messaging accounts:
In fact, throughout The New York Times article, there is continued debate between whether to blame ISIS or Al Queda. (Obviously, if ISIS had actually claimed responsibility, they wouldn’t be debating between blaming ISIS or Al Queda throughout the article.) The reality is that it might not have been either group but they need to pin it on either ISIS or Al Queda so they can declare war. If it was just homegrown terrorism exploiting Islam, they would not be able to go to war, so they pretend it’s not even a possibility.
Additionally, if ISIS was going to claim responsibility and declare open war on France, an encrypted internal communique isn’t going to achieve that.
(Another scenario is that ISIS was trying to do it in secret, didn’t want France to know, and the internal communique accidentally exposed that they were responsible. However, the encrypted message only showed that ISIS knew of the attacks and praised the attackers. They never said they did it.)
The best evidence they have linking ISIS to the crime is the encrypted internal communique which actually implies ISIS didn’t do it.
ISIS say they were impressed with the way the Paris terrorists executed the attack and applauded them on a job well done, but imply that the Paris terrorists were people they didn’t actually know.
Additionally, if ISIS had orchestrated a terrorist attack they considered impressive and successful, why wouldn’t they be patting themselves on the back for a job well done? Why wouldn’t they be cockier and more full of hubris?
If ISIS had done it, they would have been more arrogant and self-congratulatory rather than merely applauding the work of others.
Thus, I highly doubt it was ISIS.