Why in all my years did I never hear that term until about 12 months ago?
Because the two make up the declared enemy of the Left.
A. White
B. Nationalist
A+B = pure evil.
The Left’s worst nightmare is for white people to have a homeland.
It's a clever bit of propaganda that was thrown out recently and has gone pretty much unchallenged. The real definition of a white nationalist is someone who believes in a white nation, just as the definition of a black nationalist is somebody who believes in a black nation - Louis Farrakhan, for example. These are racial separatists but not necessarily supremacists.
However, anyone who is white and believes in their nation, i.e. America, is not a white nationalist despite being both white and a nationalist; nor is anyone who is black and believes in America a black nationalist. The term isn't additive, it has a separate meaning. Blurring that distinction has been an effective political smear, nothing more.