Racism is not the reason Trump is popular, it's part of why he's not popular. Obviously, a premise of the article's argument is that Trump is popular in the first place, which is demonstrably not true. He lost two popular votes against 2 candidates who were themselves already extremely unpopular. In fact he lost so badly that of the 12 major party candidates to run in the 21st century, he came in 10th and 11th, only beating out John McCain's 2008 chaotic performance against the popular Obama. And according to some polls, he may give McCain a run for his money this year.
So we shouldn't be asking why Trump is popular, we should be asking why he's unpopular. I'd argue racism is no longer popular in the US, in fact there's been majority support among white people for interracial marriage since 1995. Trump grew up and became popular in a much more racist era than today, and that harms his popularity.