General Discussion

Merely saying unpopular candidates don't account for the increase in undervotes doesn't actually prove the contention. For example what was percentage of people who disliked both Obama and McCain or Obama and Romney compared to Trump and Clinton? Because Clinton's final unfavorable rating was 52% and Trump's was 61%. Those were the two highest unfavorable ratings for candidates in history. It's far more reasonable to assume that would in fact have an impact on voting for that office. Romney's unfavorable rating in 2012 was 43%, Obama 37%. In 2008, both Obama and McCain had unfavorable ratings of 35%. These aren't numbers that should just be glossed over because you don't like them.

Also there's nothing to suggest that 70% of the undervotes would have gone to the Clinton or Harris other than the author says so.

This is still turtles all the way down that doesn't start from a firm foundation but builds theories based on ephemeral suppositions that don't hold up to scrutiny.