That's an intermediate conclusion that Huemer makes, not a premise. You can't just dismiss an argument on the basis that you don't believe in the conclusion. I'm not sure Huemer's argument is sound, but it definitely doesn't depend on whether people who consider themselves determinists believe in the truth of the conclusion. The implausibility of the conclusion is exactly what makes the argument useful and philosophically interesting, because it is supposed to make the determinist position untenable!