Having lived in Berlin and Freiburg, I can confirm this. I also find it interesting that most in southern Baden-Württemberg still refer to their dialect as "Allemanic"(as in the ancient confederation of Germanic tribes, the "Allemani" Excerpt from Wikipedia: "First mentioned by Cassius Dio in the context of the campaign of Caracalla of 213, the Alemanni captured the Agri Decumates in 260, and later expanded into present-day Alsace, and northern Switzerland, leading to the establishment of the Old High German language in those regions."). The french word for Germans, "Allemands", reflects these old tribal boundaries as well. It's easy to forget that Germany as a unified state is young, but try sometime watching someone from Berlin or Hamburg attempt to decipher a sentence spoken in the rural Allemanic dialect, it's pretty futile.(Note: this doesn't really apply to larger towns like Freiburg anymore. You have to drive half an hour or so into the countryside, and find an older person. Young people often prefer to speak Standard German nowadays).

The article is only incorrect by omission. I am not sure, but you may confusing the term "low form," that is, Low German / Plattdeutsch, with being geographically southern. It is geographically northern -- the "low" refers to elevation, not latitude.

Precisely people in Berlin/Brandenburg say "ik" _because it is Low German, which is spoken in the North of Germany, which is what the Uerdinge isogloss describes_.

The article did fail to mention the other big lines, the Benrather and the Speyer line (which is what you are referring to with Oberdeutsch dialect differentiating between "ich" and "i").

There are a lot of these lines drawn by linguists.

I agree both you and the parent, I don't think you're disagreeing, you're just talking about different things. You're talking about the historical distinction, the parent is talking about how things are nowadays.[0]

I found the article a bit disingenuous since you need to know what the Uerdingen line is and its context to understand what they're saying, since the divide isn't quite what it used to be.

[0]: http://www.atlas-alltagssprache.de/runde-2/f25c/

Hah, all this time I thought it was an Austrian thing - my late grandmother while helping me practice my German pronounced it as "Ik" and then quickly corrected herself.