It's populism. You have to see if someone's policy positions are consistent over time, that they have a specific ideal they are following, vs what they think is the most popular policies.
Granted, it's a huge pain with new politicians when they don't have that history, but I think Tulsi had a pretty clear history that showed she wasn't progressive.
There's not much to it. They simply believe that as the strife causes conflict over resources the factions will "naturally" align along racial lines. They also believe that people "naturally" cohabitate better within their own race.
This does require ignoring all of human history and the brutal conflicts that have occurred within racially homogenous regions. But I'd never accuse white supremacists of being intelligent or genuine.