How does ethnic identity threaten a mixed person?
Being mixed myself, I'd love to know why you think ethnic identity is a threat to me and others like me.
In my experience, it has been a tremendous advantage, despite the fact that I have lost friends and opportunities simply because I am not "white enough". And that isn't a guess or misread, I've been told that explicitly. It hurts, especially as a child, but knowing this happens allows me to understand the importance of exposing everyone to as much diversity as possible. Why? Because each and every time someone has mistreated me or judged me negatively based on ethnicity, it was quite apparent that they have lived a very cloistered life and oftentimes carry some sort of grudge or sense of victimhood despite their advantages. And they quite often look up to someone (e.g. their father or other adult role model) who exhibit the exact same prejudices, insecurities and victimhood.
I've also been threatened and harassed by the out-group because they thought I was of the in-group. Not a fun experience in the least. But again, it became quite clear why they behaved that way: a lack of diverse real-world experience (particularly a lack of positive experiences) combined with misguided lessons from equally misguided role-models.