His memo just seemed to me like the view a Republican holds on a polarising topic - so it's not surprising it splits into two polarised bimodal responses in my opinion.
Your supporting link says it has no substance - it's a simple meme that is wrong.
This subject annoys me because I am convinced that not only does the Horseshoe Theory have no predictive power - but also that it denies something which really is true - that political violence can come out of the Center aka from Liberalism.
This is a huge blindspot in modern political culture. Conflict coming out of the Center would be a true Black Swan event.