Depends on who you ask. What’s the region in question? What counts as a standard? Is Nynorsk codified by ISO? Et cetera.
> People write Cantonese in private of course
It‘s used plenty in public in Cantonese-native areas!
> that usage is not uniform
Yes, there is a disconnect and different ways of writing down a turn of spoken phrase. Sometimes people wouldn’t know offhand how they would even do it about a purely spoken turn of phrase.
A government regulating it by teaching it in school and actively using it in public would be a strong criteria. Alternatively, an established body of literature that serves as a model (that's how written Italian evolved).
> It‘s used plenty in public in Cantonese-native areas
Yes, that's also what I said. However, any official announcements or documents are quite unlikely to use it.
A crucial difference is that there is no standardized form of written Cantonese. There is a common set of characters used in advertising, sometimes newspapers, or in court transcripts, but that's it. For anything official, Modern Standard Written Chinese is used.
People write Cantonese in private of course, but that usage is not uniform at all since there is a lot of slang (especially swearing words) for which more than one popular way to write it down exists. Quite often the characters chosen for that purpose had different meanings originally. Some particularly novel or distinctive words and idioms are even partly written in Latin letters, like 快-D.