So many words, fujii and le pi, to provide the answer: The lateral separation buffer in Australia is not a number specified by ICAO.
The buffer is instead ... plucked out of of the arse of ... let's guess ... hmmmm ... I'm guessing someone in Australia who has the luxury of transferring the consequences of their risk aversion to someone else, backed by the mystique of aviation (i.e. the cognitive bias of punters.)
The irony is that if Dick wasn't such an egomaniac and had some capacity to heed advice, he might be able to articulate his arguments a little more convincingly for experts.