Have you often heard Australians calling black people "apes", though? There's no history of it here that I'm aware of, and it's been banished from popular culture, so it's very possible IMO that the girl didn't understand the connotations. I understand them because I'm older and lived through Love Thy Neighbour etc. But she was a 13yo from a rural area. If she'd said "f... b... ape", OK, she's no innocent. However she stated in a written apology (which Goodes publicly accepted) that she didn't know it was offensive. As I said, Goodes' reaction was perfectly understandable, but the following day he continued to milk it.
Anyway as you say this is old news, and hardly Goodes' biggest crime.