Thanks! From the blog post linked there posted on January 23: "Our primary data source is DXY, an online platform run by members of the Chinese medical community, which aggregates local media and government reports to provide COVID-19 cumulative case totals in near real-time at the province level in China and country level otherwise."
I'd surmise that it was the primary source since, at the time, the spread of the virus was still contained within China for the most part.