I wonder why there is a reference to a strong smell of "fuel" rather than a strong smell of AVGAS.
Maybe the ATSB doesn't know the difference?? Or more seriously, "fuel" is a generic term of which AVGAS is a subset. And the ATSB are reporting the comments of witnesses who will almost certainly be untrained to distinguish between AVGAS & MOGAS. I think you are reading more into this than it deserves.
Besides - the ATSB says he refuelled at Mt Gambier. I don't believe MtGambier has a MOGAS pump, so it's gotta be AVGAS.