High German???????What they mean is these people spoke, "proper, formal," German. A Swiss dialect called, "Schweizerdeutsch," is unintelligible to most German speakers.
Hoch Deutsch is spoken in Northern Germany, I.e. Hamburg. I spent the summer of 79 at the German AA range at Todendorf. It didn't take long for me to shift from the Hunsrucker dialect in the Eifel region (Trier/Mosel). That dialect is very forgiving compared to Hoch Deutsch.
It was one of the best summers of my life, hard to think of one that tops it.