In all cultures, there is an expectation that you have to provide a name for yourself that is intelligible to the culture you're interacting with, both in written language and in speech. If your name is Albert and you are going to interact with many Japanese speakers, you'll have to call yourself アルバート in writing and pronounce your name as something like "Ah roo bay toe" to fit in. If you have a name whose pronunciation depends heavily on tones, such as a Mandarin or Vietnamese name, and you are going to interact with speakers of a non-tonal language, you'll have to come up with a version that you're happy with even if pronounced in the default neutral tone that those people will naturally use. If your name is 高山, you'll have to spell it as Takayama.

Similarly, if you're going to create an identifier for yourself that is supposed to be usable in an international context, you'll have to use the lowest common denominator that is acceptable in that context - and that happens to be a-zA-Z0-9. Why the Latin alphabet and numerals and not, say, Arabic, you might ask? Because Chinese and Indian and Arabic speakers are far more likely to be familiar with the Latin alphabet than with each other's writing systems.

The article has examples about people naming themselves "Аdministrator". That's not about machine-readable identifiers, it's about display names. This entire subthread is either people missing that and thinking they're talking about login usernames and the likes, in which case I don't disagree, or people actually believing it's OK to limit people's screen names to a-zA-Z0-9 in which case I say, that's deeply imperialistic and a super shit thing to do.