I don't think for a second that the board of directors at ISIS are presiding over this elaborate empire of terrorists, controlling every p a w n (why is that banned) & planning every move KAOS style (that's from Get Smart for those of you too young). ISIS, Al Qaeda whoever, are simply the inspiration for the attacks not the actual proponents. I'd be very surprised if many of these attacks are coordinated with anyone outside their immediate circles & most would be lone nutters. Sure some of these guys in Europe have had direct contact with ISIS & some have had training, some have probably watched videos, read propaganda, been inspired by local radical Imams etc but again ISIS is simply throwing out the idea terror & will therefore claim anything & everything they can to further fantasise their cause. I don't credit them as being anywhere near as organised as they want to portray.
Does that mean this was or wasn't a terrorist attack - it makes no difference to me if ISIS claim it or not, the only thing that really matters is what was the reason for the attack. Was it politically motivated, based on what is know it appears it was. It wasn't a crime of opportunity, he wasn't demanding ransom, there doesn't appear to be anything in the victims chosen (it wasn't personal) so it looks like he was trying to make this a political statement. Maybe he was inspired by the incidents in London & thought it was time to have an attack in Australia. We'll probably never know a fraction of what was going on in this guys head.