Linearity is an excellent assumption for virtually any non-overdriven modern amplifier. THD measurements make sense for speakers, but that's not getting in the way of what the author is attempting to do here.
I don't even know where to begin addressing everything that's wrong with this article, but assuming linearity isn't one of those errors.
I don't even know where to begin addressing everything that's wrong with this article, but assuming linearity isn't one of those errors.