That's a very common usage:

"ethnic: Relating to a population subgroup (within a larger or dominant national or cultural group) with a common national or cultural tradition."

(from an online version of the Oxford dictionary: https://web.archive.org/web/20200108002157/https://www.lexic...)

I'm pretty sure that "Russian" is an ethnic group (and "German" and "Irish" and ...), and yet the "Russian" ethnic group is going to get called "white", so it's weird not to consider whites as having ethnic groups (which any statement that compares the number of "ethnic group" members to the number of "whites" is implicitly doing).

Right, but my point is that the word has multiple common meanings.

Also, to your point, the usage section of the entry encourages using more specific terms:

"Ethnic is sometimes used in a euphemistic way to refer to non-white people as a whole, as in a radio station which broadcasts to the ethnic community in Birmingham. Although this usage is quite common, more specific terms such as ‘black’ or ‘Asian’ are preferable."