From the article (which is much longer) :

In September 2016, the tiny African state was able to get Chagos back on the United Nations agenda for the first time in half a century. The following year, the UN General Assembly – over fierce resistance from London – voted to seek an advisory opinion on the issue from the International Court of Justice. The judges, responsible for territorial disagreements, were asked to determine whether the British violated international law when they split Chagos off from Mauritius in the 1960s.

The finding from The Hague came in early 2019, and it couldn’t have more explicit: The United Kingdom, the court found, had "forcibly removed" the population of the Chagos islands. Furthermore, the "process of decolonization of Mauritius (was) not lawfully completed." London, reads the court opinion, must return the archipelago to Mauritius "as rapidly as possible."

The British, though, had no intentions of complying. Even before the court opinion, then Foreign Secretary Boris Johnson had exhibited no qualms about extending the lease for Diego Garcia to 2036. Now, London ignored the International Court of Justice and, a short time later, disregarded the UN General Assembly, which voted 116 to 6 in a resolution demanding the end of British rule in Chagos. Among European countries, only Hungary sided with the UK.

Finally, London also brushed off a ruling from the International Tribunal for the Law of the Sea, located in Hamburg, which confirmed Mauritius’ sovereignty over the Chagos Archipelago in 2021.
"Ongoing Abuses By a Colonial Power"

To the surprise even of the UK’s Western partners, London continued to hold on to its last remaining African colony – with sometimes bizarre consequences. Even as new United Nations maps clearly marked Chagos as belonging to Mauritius, the Royal Mail continued to print stamps depicting the colorful marine life of "British Indian Ocean Territory" – stamps which cannot be used there because they were prohibited by the Universal Postal Association.

A recent report by Human Rights Watch notes that "the forced displacement of the Chagossians and ongoing abuses amount to crimes against humanity committed by a colonial power against an Indigenous people." The violations, says lead author Clive Baldwin, aren’t just based on lies, but also on "systematic racism."

To substantiate such allegations, Baldwin doesn’t just cite the "racist language used by senior British officials" to describe the Chagossians. He also notes the "the way Chargossians have been treated compared with other islanders of European origin living near UK military bases in the Falklands and Cyprus." In the conflict with Argentina over the Falklands, London had consistently cited the right to self-determination of the majority white residents.