I don't think anyone is trying to say that a Sportline is an AMG.
Personally, the history is the aspect that interests me, along with the fact that this degree of similarity in design would normally spawn trademark infringement litigation if there were no prior agreement between parties. Microsoft is currently suing a Linux software company because their 'Lindows' name "...could be confused with 'Windows'."

Apple and Microsoft duked it out over the look of the graphic interface that Xerox Corporation developed.

MB sued MercedesShop over the similarity in names (MB/DC lost!

), so they aren't exactly bashful in 'protecting their own'.
For all I know the 5 slashes were an MB design from their performance design center, if any, which was subsequently incorporated into the AMG logo as they began to merge. Or the Sportline suspension may have been a 'paper napkin' design by AMG and MB engineers over beer and Schnitzel after a weekend autocrossing, without any formal involvement by either company, and the 5 bars represent their favorite drinking spots.

Or it may be pure coincidence - stranger things have happened. Neither MB or AMG responded to my original queries, so who knows.
I don't try to represent my car as either an original Sportline OR as an AMG. (Which is the tail, and which is the dog? The MB engineers who came up with the factory design, or the AMG engineers who modified the design for special applications? MB existed for decades without AMG - but the converse isn't apparent!) It may seem like a waste of time to pursue this history, but on the other hand, several Ivy League universities offer PhD programs in the History of Science and Technology - this could become someone's dissertation! (I'll help them with their comps - junior author status will be sufficient recognition.) :p