Ok, first of all, let's start with the definitions of each of those terms.
the Gini coefficient just tells you how spread out incomes are, it doesnt tell you what the average income is. The US has a much higher average income than China, but the distribution of incomes around that average is similar.
Since we're talking about billionaires, we're talking about a situation where most of a country's wealth is consolidated in the hands of a small group of rich households.
Given that Australia is basically a tax haven so there's a lot more opportunity to start a business there in contrast to Brazil, the business friendliness combined with much lower corporate tax rates into ease of starting a business, available help for entrepreneurs, looser regulations, no standardized minimum wage make it much easier to establish a company than in Brazil.
Most billionaires have gotten their wealth from working in the financial sector or through generational inheritance.
For a billionaire to be a billionaire he/she either has to be self-made or be subjected to some kind of inheritance, and for you to make such a bold claim that there is no relation to how developed a country is and how distributed is its wealth in relation to being a billionaire is something completely smart or incredibly stupid.
Let's assume that what you had said was right, that Australia has 6.7 times more billionaires than Brazil per capita considering that AU's population is 15million and Brazil's is 202 million. According to a research, 47,7% of Brazil's billionaires acquired their wealth through inheritance, 21,5% through self-made company founders, Australia has 33 billionaires, of which 22 are self-made, which means roughly 66% of them are self made, and thus relied on the opportunities given by the country to establish a private sector, also 11 of them (33%) are immigrants, and Australia has the largest minimum wage of all the OECD countries,
If you take into account that the gini coefficient of Brazil is very low compared to other countries such as AU, and that income inequality is very dangerous when paired with a low median income because it'd make up for a pretty unhealthy economy that'd lower the demand-driven feedback loop that forces supply to grow, which in turn would reduce employment, which in turn would weaken demand, the result would be a much less foreign investment and if the domestic economy isn't doing well, there'd be no chances of starting business.
So how can you say that wealth distributed and the country's development is not relevant to this context?