"IT'S ALL IN THE NAME Because 'International Organization for Standardization' would have different acronyms in different languages (IOS in English, OIN in French for Organisation internationale de normalisation), our founders decided to give it the short form ISO. ISO is derived from the Greek isos, meaning equal. Whatever the country, whatever the language, we are always ISO." https://www.iso.org/about-us.html

But I'd tend to call BS - if it wasn't ever an acronym, why capitalize all three letters?

The google ngrams are also suggestive:

https://books.google.com/ngrams/graph?content=international+...

The Greek ισο "iso" does mean "equal". I do not know if that influenced the naming of the organization, though.

You see this word in many English roots, such as isotropic, isometric, isomer, etc.