The research paper is available online with a description of the methodology.  It only looks at CCPCs, so it appropriately excludes sole proprietors, to answer your question.  
There are two limitations that skew the data in opposite ways: 
(1) It only identifies income splitting if the participants share the same address, which would cut out some where adult children live on their own b/c they can't track it with the data they have.  This minimizes the potential impact.  
(2) It assumes that all the dividends are for income splitting.  So included in the 1/8 might be businesses where the family member works there, and their tax is not affected by this rule.  So the 1/8 might be too high b/c of this.

I'm not surprised the impact is low, and skews to high earners.  The tax benefit is bigger when you have family members in different tax brackets (and the bigger the difference, the better the tax benefit), which isn't as common anymore.