I have no idea whether or not the origin given on this site is correct. But the fact that the way the Wiradjuri language is written today does not include the letter "e" does not by itself show this has to be incorrect. People write down names and words from other cultures using their own alphabets all the time, and it is certainly possible that
Jiemba could be how some English speaker "heard" a Wiradjuri name and wrote it down.
I hope that someone who knows will explain where the information about this name on this site came from.
Jiemba is not included in "The Great Australian and New Zealand Book of Baby Names" by
Cecily Dynes, which is the one book I own that specifically includes some names said to be of Australian Aboriginal origin.