Well, it’s different if you look into the German Wikipedia Hochdeutsch and what is stated there is much more accurate than the information in the English Wikipedia.
Let’s just review a few sentences:
High German (in German, Hochdeutsch) is any of several German dialects spoken in Germany, Austria, Liechtenstein, Switzerland, and Luxembourg (as well as in neighbouring portions of Belgium, France (Alsace), Italy, Poland, and Romania (Transylvania) and in some areas of former colonial settlement, e.g. in Namibia).
Well, actually “hochdeutsch” is not spoken in these areas, but dialects are. “Hochdeutsch” is an artificially constructed version of standard German in writing and speech. It is recommended to be used in all these areas to prevent babylonic diversion of dialects. “Hochdeutsch” relates to German as a “presciptive English grammar” relates to “English Grammar”.
“High” refers to the mountainous areas of southern Germany and the Alps, as opposed to “Low German” spoken along the flat sea coasts of the north.
Well yes, this is correct in some sense, but it is very rarely used in this meaning. However “Niederdeutsch” und “Mitteldeutsch” are often used in this sense to refer to German geography.
The German term Hochdeutsch is also used loosely, but not by linguists, to mean standard written German as opposed to dialect, because the standard language developed out of High rather than Low German.
Well, this is wrong for most parts. “Hochdeutsch” is not used loosely to refer to standard written German, but instead, this will be the only meaning of the word for many Germans. The German word for “standard written language” is “Hochsprache”, hence “Hochdeutsch”.
However, it is correct that “Hochdeutsch” was developed from southern German, but if you ever heard an southern dialect and compare it to “Hochdeutsch”, you would not believe it. Mainly “Hochdeutsch” developed in official writing and art, not in spoken language. It is only now in modern times with radio and TV, that “Hochdeutsch” becomes to a widely used spoken language.
This were only my comments to the first three sentences of the Wikipedia article, I did not read any further.
cu