I think you've misunderstood the difference between acronyms and initialisms. Something can be both, or just one and not the other.
a word formed from the initial letters or groups of letters of words in a set phrase or series of words, as Wac from Women's Army Corps, OPEC from Organization of Petroleum Exporting Countries, or loran from long-range navigation.
- a name or term formed from the initial letters of a group of words and pronounced as a separate word, as NATO for North Atlantic Treaty organization; an acronym.
- a set of initials representing a name, organization, or the like, with each letter pronounced separately, as FBI for Federal Bureau of Investigation.
- the practice of using initials or forming words from initials.
As you can see, WAC, OPEC and NATO are both acronyms and initialisms because they're a bunch of initials that you can pronounce as a word.
FBI on the other hand is a bunch of initials you cannot pronounce as a word; it's an initialism but not an acronym.
And finally, loran is pronounced as a word, but isn't only taken from initial letters; it's an acronym but not an initialism.
So for IO:
- If you pronounce IO as a word (like Greek "io"), it's an acronym.
- If you pronounce IO as letters I-O ("eye oh"), it's not an acronym.
- In both cases, IO is an initialism.