Why? Judging from the photo his pack looks fairly light and minimalist. His clothing seems robust. Effective-looking head cover. Boots are probably heavy but nailed. Typical and practical working clothes of the time it seems to me.

I wonder how many supply stops he had to make and how this was funded. I doubt a pair of boots would last a fraction of this walk.

I'm not sure. I want to believe in that kind of perseverance and exploration, but to give a glimpse of the "Aidan" here's an excerpt for the end part of the book:

"Little is known about Aidan de Brune following the conclusion of his walk. We catch a glimpse here and there. On 28 March 1933 an article by de Brune appeared in the Wagga Wagga Daily Advertiser as one of a series of articles about Australian novelists. The article was about de Brune himself, in which he outlined his life. It was mostly fictitious. As we know, de Brune was actually Herbert Charles Cull and he did few, if any, of the things mentioned in the article." .. so if he's known to embellish or fabricate details...?

What is supposedly missing from his rig so that you repute it fit for a walk?

The light load is a great sign he knew what is important during endurance adventures.

Funnily enough I had the same sort of "doubting people" while I walked across Spain and Portugal with a 20 liter day pack.

What special 'rig' do you think you need to walk?

Food? Water? Unless he was close to civilization at all times and just stopped into taverns to eat.

The book says there were stations every few days all the way around.

I can't speak for non Queensland parts of that map.In the 1900's there was more than a week of walking between even the most basic of civilization (Between mackay and townsville, the A1/Bruce started construction in 1930)

Hardly “unassisted” then.

Unassisted just means you don't have a dedicated support team.