This was a shit-tier human being mentioning ISIS to scare people and make himself seem bigger than the shit stain he was. ISIS of course took credit. Had he not been muslim at all it might have been just another mass shooting where some scumbag seeks attention. Based on this guy's story though, I don't think ideology was the sole motive, he almost certainly wasn't specifically recruited, trained, and equipped by ISIS to perform this attack.
ISIS is banking on Muslim individuals that are shit people to begin with to do this kind of thing in their name. They're essentially trolling us in the hope that we formally wage war with them. As it is they aren't having a great time.