So, they're going to be "sovereign" over the whole archipelago, but only as long as they don't exercise their sovereignty over the largest island in it which constitutes more than half of its total land area (30 km² of 56.13 km²).

From today's Joint statement between the governments of the Republic of Mauritius and the United Kingdom of Great Britain and Northern Ireland concerning the Chagos Archipelago, including Diego Garcia:

Today’s political agreement is subject to the finalisation of a treaty and supporting legal instruments, which both sides have committed to complete as quickly as possible. Under the terms of this treaty the United Kingdom will agree that Mauritius is sovereign over the Chagos Archipelago, including Diego Garcia. At the same time, both our countries are committed to the need, and will agree in the treaty, to ensure the long-term, secure and effective operation of the existing base on Diego Garcia which plays a vital role in regional and global security. For an initial period of 99 years, the United Kingdom will be authorised to exercise with respect to Diego Garcia the sovereign rights and authorities of Mauritius required to ensure the continued operation of the base well into the next century.

The treaty will address wrongs of the past and demonstrate the commitment of both parties to support the welfare of Chagossians. Mauritius will now be free to implement a programme of resettlement on the islands of the Chagos Archipelago, other than Diego Garcia, and the UK will capitalise a new trust fund, as well as separately provide other support, for the benefit of Chagossians.

It will also herald a new era of economic, security and environmental partnership between our two nations. To enable this partnership the UK will provide a package of financial support to Mauritius. This will include an indexed annual payment for the duration of the agreement and the establishment of a transformational infrastructure partnership, underpinned by UK grant funding, to deliver strategic projects generating meaningful change for ordinary Mauritians and boosting economic development across the country. More broadly, the UK and Mauritius will cooperate on environmental protection, maritime security, combating illegal fishing, irregular migration and drug and people trafficking within the Chagos Archipelago, with the shared objective of securing and protecting one of the world’s most important marine environments. This will include the establishment of a Mauritian Marine Protected Area.

Nice touch making a new "Marine Protected Area" in the process; the current "Chagos Marine Protected Area" was created entirely to, well... lets let this 2009 US diplomatic cable published by WikiLeaks explain:

1. (C/NF) Summary. HMG would like to establish a "marine park" or "reserve" providing comprehensive environmental protection to the reefs and waters of the British Indian Ocean Territory (BIOT), a senior Foreign and Commonwealth Office (FCO) official informed Polcouns on May 12. The official insisted that the establishment of a marine park -- the world's largest -- would in no way impinge on USG use of the BIOT, including Diego Garcia, for military purposes. He agreed that the UK and U.S. should carefully negotiate the details of the marine reserve to assure that U.S. interests were safeguarded and the strategic value of BIOT was upheld. He said that the BIOT's former inhabitants would find it difficult, if not impossible, to pursue their claim for resettlement on the islands if the entire Chagos Archipelago were a marine reserve. End Summary.

I wonder how the "treaty will address wrongs of the past"; somehow I doubt it will involve any mention of the CIA torture site there.