tpo As far as I understand, Sozi has moved on to be an inkscape plugin.
As far as I remember, the implementation by polch was an alternative to the official Sozi presentation editor, which was an Inkscape extension at the time (2013). It allowed to create Sozi presentations from Dia without using Inkscape.
Sozi integration in Dia is not officially linked to the Sozi project. I don't know if it is used today, but I guess it is very outdated.