I don't think Trump really is a supremacist, and I certainly don't believe you are.
But Trump has shown himself willing to do and say absolutely anything to further his cause. He appears to see white supremacists as a constituency ignored by both parties (for good reason). And this isn't the first time he's associated himself with them. He's also retweeted some supremacist stuff, and there was the David Duke mess.
Duke, by the way, commented enthusiastically on the problematic tweet about Hillary.