First of all let's just talk about the rarity of this type of attack they're basically saying that somebody smuggled a bomb on to the plane possibly in the baggage area. A bomb attack like that has not been successful since 1989 every other attempt on an airliner has either been foiled or done by different methods like improvise shoe bomb or a a liquid bomb where somebody is actually carrying it onto the plane. There were no reports of anybody but Russian nationals on that plane and there are no suspects of suspicious people that might have smuggled it on there and detonated in a suicide attack meaning that the attackers had to have planted the bomb on the plane and and not been on the plane themselves.

This is not how al queda attacks, and we can assume that ISIS being comprised mostly of former members of alqueda would have given up on this style of attack a long time ago. this is not how it's been done in the 21st century even on 911 there was no bomb on the plane plot. Bomb on the plane plots are very Hollywood style plots but very rarely happen it's usually hijackers like a 911 who you used the plane is a weapon or a suicide attack where they're smuggling it on and they're going down with the plane.

I think this operation was put together in haste due to Russia pushing the envelope by starting airstrikes in Syria. The CIA needed to strike Russia as fast as they can so they put a bomb on a plane and told ISIS to take responsibility. But then came the claim of responsibility by ISIS which was incorrect in saying they shot it down. And they said that because they know that's the only way ISIS knows how to take down a plane. Terrorists have given up on trying to place a bomb on a plane, they're all about the suicide attack in the 21st century. So the CIA told ISIS to take responsibility for the downing of the plane but didn't tell them how they were going to do it.

This was the easiest course of action for the CIA. A bomb smuggled onto the plane is so much easier than trying to fake a suicide attack. And when ISIS said they shot it down its because that's the only way they were capable of downing a plane.