There is no evidence to even slightly suggest that they were in a CB. In fact, based on the statements in the report (the call to the cabin, the 12° left turn, the decel to M0.8) they knew very well what was going on with the weather and were steering around the worst bit. IMO they were obviously not in a CB, but in cloud in bad icing conditions
But there was convective WX around and we don't know what the tilt setting was - they could have been dangerously overscanning the storms, leading to a false impression that the flight path choosen (10 left) was clear:
http://http://www.weathergraphics.com/tim/af447/