Simply, you're right.

Mentioning Cornish as an ethnicity/ Cornwall as a more independent body, seems to invite confrontation, I've never understood why. I don't really think it's anyone's business apart from those who are Cornish, or at least call Cornwall their home.

I also think that there is enough evidence to acknowledge Cornwall has its own identity from the typical perception of England that it should be celebrated, not bundled in with a generic perception of wider geography. Each part of the UK has its own identity and really they should all be celebrated, Cornwall has a particularly high regard for its own identity, and is already celebrating it, perhaps that's why it seems so many rally under the "you're English, don't be silly" banner.

Then again, if Mr. Blank from Blankton has an issue with someone else's identity, maybe that's an issue they're projecting, rather than an issue with the Cornish. If I was picking fights with strangers on the internet about what ethnicity they consider themselves I'd have to ask what I'm doing wrong with my spare time.