I would say referring to a person or set of people as unjust is a little bit of a category error, like saying "gravity is purple".
The correct thing is probably to chalk this up to lazy writing (it's not the only grammatical error in the piece) and interpret the author as actually meaning "A society or legal framework that allows rent seeking behavior is unjust and dysfunctional."
If we can agree approximately on criteria for "justice" and "dysfunctionality" (probably involving fairness and sustainability in one way or another) then this is a claim that's amenable to rational discussion and debate, and maybe even a little bit testable empirically.
The main problem I have with this is that it's not very interesting and doesn't solve anything.
But that is about the only difference between how the modern gears of power operate versus how they did millennia ago. Rather than just kill your brothers and claim the throne and execute the dissidents and having your firstborn son inherit the crown from you, your children inherit ludicrous wealth that translates into the same power to buy the violent arm of the state for your own ends.
Except now the peasantry cannot just storm the castle, behead the king, and replace him with another. There are kings from across the globe that control your society through its economy, and the present incumbents of radically concentrated power will argue everyone is king when they own wealth in the global capitalistic system.
That, in practice, is pure deception[1], but you will have people by thousands at the bottom congregating to defend those off the chart, despite the ones at the top constantly using their power to harm all those below them.
If we did have what the rich claim we do - democratized distributed power - we would need dramatically less wealth inequality, because that wealth, and the distribution of it, represents who has authority in the global capitalist economy. That is not an argument for socialism, communism, fascism, anything political. It is a reflection of tautological truths about having a global capitalistic market system that determines who is powerful by way of capital and fiat ownership, and that if you want to have a more democratic humanity you must first deal with the fact that money is power, and when power is aggregated into the hands of very few, you don't have democracy.
We are not at the point to argue practicalities, or dogma, or anything else beyond understanding. What we need is just for near universal acknowledgement that this paradigm exists, is how the world operates today, and that you probably would want to do something about it because we can already universally agree we don't like tyrants, and that we should not like tyrants in any form they take, even the ones that are not immediately and obviously apparent to most.
Except inherited wealth is at all-time lows.
If you look at the richest people in the world today, the majority of them made their own way into those ranks. They didn't inherit it from anyone.
In reality, money might not stay in one families direct hands, but upward mobility is at generational lows in the west, and in particular the number of successful new businesses, and those which elevate anyone out of the working class into either middle or upper class, are diminishing significantly[1].
[1]: https://www.imf.org/external/pubs/ft/sdn/2015/sdn1513.pdf
If you take 30% of everyones income, the working class loses standard of living and upward mobility while the capitalist class reevaluates their investments to insure that, despite a 30% annual expense, you are growing your wealth regardless.
It is also a dangerous precedent. If your intention to tax sufficiently high to start peeling back the democracy distorting wealth inequality, the wealthy will just flee you, and you must remember - as they exist today, they are the arbiters of society, and dictate who lives and dies through economics. If anyone outright attacked them like that, you should expect a war in return.
If that could be made to work and the pieces set up to work, would it be a solution?
http://paulgraham.com/disagree.html
Let me try and do a little better: I would argue that this kind of rhetoric is not significantly different from social movements in various countries of the 20th century, the end results were not quite as benign as its supporters anticipated: 'During the land reform, a significant numbers of landlords and well-to-do peasants were beaten to death at mass meetings organised by the Communist Party as land was taken from them and given to poorer peasants.[169] The Campaign to Suppress Counter-revolutionaries,[170] involved public executions that targeted mainly former Kuomintang officials, businessmen accused of "disturbing" the market, former employees of Western companies and intellectuals whose loyalty was suspect.[171] In 1976, the U.S. State department estimated as many as a million were killed in the land reform, and 800,000 killed in the counter-revolutionary campaign.[172]
Mao himself claimed that a total of 700,000 people were killed in attacks on "counter-revolutionaries" during the years 1950–52.[173] However, because there was a policy to select "at least one landlord, and usually several, in virtually every village for public execution",[174] the number of deaths range between 2 million[174][175] and 5 million.[176][177] In addition, at least 1.5 million people,[178] perhaps as many as 4 to 6 million,[179] were sent to "reform through labour" camps where many perished.[179] Mao played a personal role in organizing the mass repressions and established a system of execution quotas,[180] which were often exceeded.[170] He defended these killings as necessary for the securing of power.[181]' [typo edited] https://en.m.wikipedia.org/wiki/Mao_Zedong