"Mauritius" has no native population. The people there are the descendents (presumably) of early slaves and folks who were growing sugar and what not that Dutch IIRC and the Brits brought in.

No place on earth has a "native" population. Every place on earth was originally inhabited by people from somewhere else. The rules of international law governing decolonization and independence - allowing self-government by the then native inhabitants of an area rather than a colonial power on the other side of the world - don't depend on whether there was a population there prior to or after the first "discovery."

The anti-colonial laws are a stretch. They're meant to cover places like India, French Indochina and other spots that obviously had a native population when the Colonial powers took them over. Mauritius has none of that.

You're welcome to your opinion, and the British certainly argued that Mauritius had no legitimate claim to the area. Though I'm pretty sure they didn't make that specific argument - it's not exactly a great look to argue that because these peoples' ancestors were brought there in the 1700's, rather than being conquered in the 1500's, it's okay for Britain to rule them instead of allowing them self-determination and independence. There's a reason why Mauritius is independent, rather than still a colony of Britain, despite not having a pre-existing native population.

Anyhow, the UK lost the argument, both at the ICJ and the UN. And the diplomatic and reputational costs that comes from being in continuous violation of an ICJ ruling and a UN Resolution continue, regardless of what they argue.