When people say "he/she looks ethnic", they don't mean white. This is a common colloquialism. He doesn't mean that white isn't an ethnicity -- he means that it'

When people say "he/she looks ethnic", they don't mean white. This is a common colloquialism. He doesn't mean that white isn't an ethnicity -- he means that it's the default setting in the US. The poster is not defining this in a way nobody else does; to the contrary, they are defining it in a ubiquitous way.