I lived in Italy many years.... windsurfed Lake Garda many many times.
I took a look at the Italian forum you linked to.
What exactly happened is not clear yet but here is what I gleened from the Italian forum.
It apears that he was found drifting by a windsurfer. His kite was bobbing at the edge of the wind window. The windsurfer could clearly see that there was nothing he could do as the kiter was already dead.
He went in and called the authorities.
He then went back and stayed with the body untill the boats came out. He freed the kiter from the kite that apeared to have no tangles at all. He just had to release the chicken loop and leash.
Apparently he had a board leash and a helmet. He normally rides with a life vest, but for some reason did not have it on.
I'll look around and see if I can get more information.
First of all this happened in the southern area of the lake, not the "thermal wind tunnel" in the northern part of the Lake.
Regarding the weather the forcast was for NW winds, but there was a very sudden
"groppo" from NE. A "groppo" is sudden increase in average winds, normally a doubling, but with stronger shifty spikes. A groppo normally lasts 5 to 10 minutes. They are quite common over the lakes in northern Italy. In this case it went from NW at about 16 kts to close to 50 kts and these are averages. The spikes when the groppo hit could have been significantly higher.
What makes these particularly dangerous is that they are sudden and involve a direction change of the wind that is more than a 90 degrees shift and without the wind dying momentarily.
Basicly if you are kiting you kite goes from being at the edge of the wind wind to right in the powerzone.... not becasue the kite moved, but becasue the wind window moved
When the wind suddenly changes directions the two winds basicly "butt heads" and where they meet there can be hellish up or down drafts.
I was caught once in a groppo on lake Como. The lake suddenly turned to foam and both my board and sail blew away. I was close to shre but it was a nighmare getting back to solid ground despite wearing a life vest. When I got to the beach a car had been turned over..(a light Citroen Dyane)