But the Russians are such disinformation artists you never know what to believe coming from them. They say things they want others to believe. This doesn't mean it's the truth, or that they themselves actually think or believe it. For some reason they might simply want the world to think that they think it wasn't a bomb. They are masters at the complex game of chess, thinking many moves ahead. We in the US, unfortunately, have a hard enough time playing checkers.
That is why I believe that Russians desperately wanted it to not be a bomb - their media was constantly talking about technical failure and they showed very hostile reactions to British statement that Brits believe it to be a bomb and almost insisted that it was a technical failure of Russian plane.
I believe it is because Putin does not want Russian people to pressure him into war. So far he can do the easy thing of flying safe sorties and dropping bombs and declare it a victory, but if he were required to put boots on the ground then the weakness of Russian military would show and people would get upset about Russian casualties.