The cartoon in the link that you have provided is clearly asserting that imperialist policies have been unjust and criminal and that the beneficiaries of those policies today cloak themselves in the ideas of justice while denying justice to those who have been hurt.
The cartoon asserts that the earliest European settlers were morally equivalent to pirates. It says that they stole the land from its rightful owners. It then shifts from historical times to the present day. It argues that the system of justice that has been set up by the people who colonized protects their ill-gotten gains while claiming to be just.
This is a very scathing indictment of “imperialist policies and practices.” It argues that these policies and practices have been unfair to the First Nations people. They have stolen the land that belonged to the First Nations and have set up systems that claim to enforce justice even as they are really protecting the resources that the imperial powers stole.