Where was the outdated message template posted then?

You'd have to ask the western media outlets which have been circulating this alleged ISIS confession that question.

Regardless, even if some official ISIS channel did come out and claim the attack, a) we know that ISIS is controlled by CIA and Mossad so they can just get the order from their handlers to go ahead and falsely claim it; it means nothing. And b) the apprehended perpetrators of the attack have not claimed to be acting on behalf of ISIS, and it's not that we should take them at their word but that is just not how ISIS followers behave. They want the world to know it was ISIS. They also want to become martyrs, they don't typically try to make an escape.