Given that Madagascar was seemingly settled by Polynesians, I'd be amazed if Europeans and their slaves were actually the first ever inhabitants of Chagos, but that aside, if they were, then the Chagossian descendents of those first slaves would be, by definition, the indigenous people of the Chagos Islands.

Edit - and will people stop buying .io domains. They might look cool but it is straight up theft.

https://www.thewebmaster.com/hosting/2016/feb/27/io-tld-top-...

https://gigaom.com/2014/06/30/the-dark-side-of-io-how-the-u-...

https://www.independent.co.uk/news/uk/politics/government-ac...