[mmarq]

> it's because they are bad at reading/writing as a skill and might make e.g. spelling mistakes.

Reading and writing are part of language proficiency. If someone struggles to understand complex written texts, then their command of the language isn't as strong as that of a proficient C1/C2 user, even if they are a native speaker.

> they have to talk about a topic they don't know.

When I took my C1 exam, I had to read and discuss a text about polymers used in aircraft wings. The definition of C1 describes someone who can understand complex texts and use the language in academic and professional contexts. Many native speakers are not even close (eg I know a number of people who were born and raised in Italy who don't understand all the moods and tenses, so I have to simplify my language when speaking with them, as I would with a foreigner).

What you have in mind is “sounding like a local” or “not having a foreign accent” or “knowing most of regional idiomatic expression”.

[froh]

yes.

indeed.

C1 is high school.

C2 is academics.

see sibling comment for links: https://news.ycombinator.com/item?id=48828753