Interestingly, whilst Wikipedia does say that, the language in RFC 1591 (Domain Name System Structure and Delegation) only says:

There are a set of what are called “top-level domain names” (TLDs). These are the generic TLDs (EDU, COM, NET, ORG, GOV, MIL, and INT), and the two letter country codes from ISO-3166.

Likewise, in ICANN’s PRINCIPLES FOR THE DELEGATION AND ADMINISTRATION OF COUNTRY CODE TOP LEVEL DOMAINS, they say:

‘Country code top level domain’ or ‘ccTLD’ means a domain in the top level of the global domain name system assigned according to the two-letter codes in the ISO 3166-1 standard

In neither case do they actually limit two letter TLDs to being country codes, they only state that all country codes in ISO 3166-1 are ccTLDs. In the RFC, the author does suggest it is unlikely that any other TLDs will be assigned, but this has obviously been superseded with the advent of gTLDs. Thus I still consider it likely that the .io TLD will simply transition to being a commercial one, rather than a country one.

Having said all that, it’s entirely possible I’ve missed some more recent rule that tightens this up and only allows two letter domains from ISO 3166-1. If I have I’d be glad of a pointer to it.