I have lurked for quite a while here, but I might have something pertinent to offer.
It seems that the pilot was in a climbing turn and that he started it in VMC before entering cloud. At that point he is likely to have looked down towards his instruments. This could well have triggered the Coriolis Illusion. This is not just disorientation and it is not the leans, but a full-blown sensation of spinning about all three axes.
This once happened to me. I was taking off from a frigate in a Wasp at night, The cloud base was forecast at 300 feet, but it was actually no more than 100. As I was turning and climbing away from the ship visually I entered cloud and immediately looked down to the instruments. I was experienced and current and I knew straight away what was happening, but it required an immense effort of will to ignore what my senses were telling me and not put in any control inputs. It probably lasted less than ten seconds, but felt like a lifetime. It really could have been a killer.