Interesting... many thanks for your translation.
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.
One possibility is that at 12000ft+ he was suffering from hypoxia. Years ago, I flew with an instructor who said he didn't need oxygen, at FL120
After a while he could not read the altimeter, and decided it might be a good idea to use oxygen after all...