Ethnicity totally can be part of an identity, but that doesn't mean you have to hate anyone.
Don't misunderstand "closing off borders". Making the people stay is not okay. Not letting the people that left back is also not okay. Not letting in a different ethnicity? Arguably racist, but I wouldn't count not letting them into your country as oppression. I'm a christian, so I can't visit Mecca, yet I don't feel oppressed.
That's why I said "long established" and that's why I pointed out that those aren't many. I don't think the violence you'd have to use to establish an ethnostate is in any way acceptable. If the ethnostate already stands though, you may use certain tools to preserve it, which are not many and are usually "not enough".
My take wasn't that ethnostates are good, but that they are not inherently bad either, it's just the tools usually used to establish and preserve them that we must condemn.