I'm guessing it's nonsense - if the Islamic State had the capability to shoot down an passenger airplane at cruising altitude they would have used it by now. They're just taking advantage of the tragedy imo.
I doubt they could shoot it down, but manage some form of sabotage or bomb? Perhaps.
Fact is, so far, nobody knows?
ISIS, or rather someone that might or might not really be ISIS, claims they did it, and they certainly could claim motive, but then, they could claim responsibility out of sheer opportunism. If they cause fear and economic loss, without lifting a finger, it's a win. If they really did it, given Russia's recent actions in Syria, it's a win. Either way, they could be claiming it. Or it could be a hoax (the claim, not the crash).
Until actual facts start emerging from the investigation, all we seem to know is there were flight difficulties, it came down, and the loss of innocent life is tragic.
There isn't much constructive to be said yet, so it comes down to commiserating with those that lost friends or loved ones, and wishing RIP for the lost.