Populism is a completely relative term that describes the "popular" opinion of a group of people. Many current populist ideas are also favored by libertarians: auditing the FED, ending corporate welfare, leaving social/moral legislation to the local level, et cetera.
Libertarianism focuses on the rights of the individual, which sometimes overlaps with the populist thought of the day and sometimes is completely different, since every human is unique.
The main issue is that when the Political Matrix refers to populism, it is essentially talking about Bryanite populism. Rhetorically speaking, extreme conservatism/liberalism/libertarianism can be very populistic, despite having little in common with the PM definition of populism.