Hard to agree or disagree, but I am not convinced, because a reasonably experienced pilot is not going to be flying in VMC (in which, at high altitude, visibility might be 50-100nm) and then suddenly decide there is cloud ahead and start doing crazy maneuvers like that.
Secondly, he was Instrument rated, and one would assume that if he had gone into cloud inadvertently, it should have been a controllable situation, unless of course something else was distracting him/onset hypoxia/or a passenger situation
Actually what the report says is that the pilot was likely flying above clouds and then started a right turn likely in order to fly down the valley and rejoin the Sion CTR entry point. Imagine a right 270 descending turn to intercept the green arrow that depicts the webcam view.
So it may well be that he was aiming for a hole in the clouds (as he had to descend down along the valley towards the CTR) and he misjudged his turn (note the report says that starting from point "A" the rate of turn increases and the sink rate as well). The pilot knew full well there were clouds ahead and below - it seems he decided he wasn't going to make the intended hole through them...
The report suggests that while turning and descending he might have inadvertently entered IMC and considering the terrain he was flying over he presumably pulled up to regain visual references and maintain terrain clearance (i.e the distraction mentioned above might have been the fear of smashing into a peak). The sharp pull up (average rate of climb 5'400 fpm between points B and C) might then have resulted into a stall.
The report further says that the PIC had only 60 hours VFR on type, so probably he wasn't very used to mountain flying and tactical planning in that particular aircraft.