There are only some dozens of people with the name Immelmann living in Germany. As well the name Imelmann is very, very seldom.
The name Immelmann is reportedly derived from the word Imme = bee.
Germans with names with "mann" are often dropping the second "m" if they emigrate to English-speaking countries. So in fact the forefathers of Trevor Immelman could have been carriers of the name Immelmann in Germany.
By the way a number of sources spells Trevor´s name really Immelmann!
I wonder if the journalists did already ask him for a potential relation?