"If any 'claim of responsibility' emerged hours or days after a terrorist attack it would be treated with the utmost suspicion. Even if transmitted directly to them, news networks would notify the authorities of course, but they might not even report it publicly. Unless a phone call or fax was received at the moment of breaking news, some times even minutes before, the information was deemed to be zero-credible or less than zero, more likely than not the work of a crank. And news sources were generally averse to being cranked."
Some times yes, some scepticism is in order. In the case of the downed Russian plane, I'd even agree. But in this case, no. If you can't tell the difference too bad for you. It's obvious the IS claim is genuine, at very least it's a similar islamic group. Who do you want to blame? The Russians?