Does this line of reasoning make sense in terms of economic agents, even if implementation is highly unlikely?
The Gini coefficient is an imperfect but reasonable measure of income inequality within a country.
One reason that governments avoid increasing income tax on high earners is that they will move to another country with lower taxes.
Therefore, if all countries agreed to a certain level of high-income tax ("we will all tax our highest earners at least 45%"), there would be no incentive to move and all countries could tax their high earners more.
Governments could use the taxes to pay for labor, or basic income, which should improve the Gini coefficient for income in their country.
I've never quite understood claim 2, because firstly you are taxed on income not wealth, and secondly I assume you can't simply transfer huge amounts of money between countries.I've focused on income.
Although claim 3 seems unlikely, in 2021 the G7 reached an agreement on taxing corporations:
The United States, Britain and other large, rich nations reached a landmark deal on Saturday to squeeze more money out of multinational companies such as Amazon and Google and reduce their incentive to shift profits to low-tax offshore havens.