That's a good mechanism too.

I've literally had an instance where a woman who was mixed race was told to "pick one."

Just to be clear how Leftist racialists view the world. They don't exactly see people as "mongrelized" like the old Progressive Racists did because they were genetic determinists. The modern Progressive Racialism is based on self-identification. They are demanding you choose a racial-political affiliation. This is also how black people can be "white supremacists". They are blacks with a "white false consciousness" who've "affiliated" or "allied" with "Whiteness".