It's because the alt-right movement came from a lot of basement dwelling cac weirdos, many of whom are obsessed with anime. So some of them have an Asian fetish and will try to justify it with "they have high IQs" or whatever. But I'm not sure if most alt-righters are like this, just some of them. Either way, they can all go die in a fire.
Then you have a lot of white guys who are racist but not full blown white nationalists who will date/marry Asians and Latinas. I know a racist Trump supporter (redundant, I know) who's dating a Salvadoran immigrant.