It doesn't matter who "owns" it (the country probably outsourced management of it to another entity, I assume); the domain exists because it exists as an ISO country code. When the country is no longer a country, and the IO country code is removed from ISO 3166-1, the justification for the domain existing will be gone. The article is saying that per current IANA policies, that should trigger the domain to be retired over a period of several years.
Personally, I do find it highly unlikely the domain will go away. They'll do something to keep it around. As the article states toward the end, "The IANA may fudge its own rules and allow .io to continue to exist. Money talks, and there is a lot of it tied up in .io domains."
ISO 3166-1 defines codes for "countries, territories, or areas of geographical interest". When the country is no longer a country, the country it's becoming part of might very decide to treat it as something still deserving an ISO code and thus a ccTLD. (and such a status makes sense for pure geographical reasons, its >2000km from Mauritius)
I believe the reasoning is that the list was originally used for post, so far-flung regions of a country may have their own codes, even if they're not politically separate. GF, French Guiana, is a good example. Politically, it's merely a region of France, but it still gets its own code.
If Mauritius decided to used a variant of Chagos/Chagas (so probably CS, since that's the only available code that still somehow fits) then IO will probably be ejected from ISO 3166.
The article addresses this: what happened with .su is part of what caused ICANN/IANA to update their policies to not have defunct country codes stick around.
Additionally (and more importantly), SU is still reserved in the ISO 3166 list (https://www.iso.org/obp/ui/#iso:code:3166:SU), so if United Kingdom somehow convinced ISO 3166 MA to reserve IO then it will be messy.
Ironically, there's some unintended truth in their reference (i.e. to all intents and purposes the KGB hasn't gone anywhere and basically is still around, just renamed/reorged) but that's an entirely different thread.
KGB was split to what is now SVR (ex 1st main department) and what is now FSB (2nd main department), with some other organizations taking remaining roles.
These days however, the Russian intelligence landscape is more complicated, with multiple semi-private organizations, informal groups, and other stakeholders.
Personally, I do find it highly unlikely the domain will go away. They'll do something to keep it around. As the article states toward the end, "The IANA may fudge its own rules and allow .io to continue to exist. Money talks, and there is a lot of it tied up in .io domains."