Chagos should get direct control of .io, but it is a weird political fight.
One awareness campaign: http://www.thedarksideof.io/
I don't know, it's a problem for politicians and standards bodies. Even if not "British", Chagos is still inside the "Indian Ocean", so the reason for the country code remains.
It's fine if people want to raise awareness to Brits behaving badly. But saying .io should go to those people is misleading.
That is much closer to the ccTLD original intent than any of the British territories have seen (.io, .vg, etc). It's not misleading to suggest that the territory control their own ccTLD's destiny, given that was the original presumption of the early IETF and many of the original NICs.
Of course it's not a "natural" resource as a digital artifact of the internet economy, but that doesn't mean the ccTLDs weren't intended to be a resource to a specific locality, and that that specific locality shouldn't most control or best benefit when that ccTLD is exploited by foreign interests find a different use/meaning/domain-hacks for that TLD.