I don't think there's a whole lot to understand in this study; it's pretty wildly extrapolated and its savings numbers come from spherical-cow projections of what single-payer would actually do (somehow, it would drastically cut pay to doctors without chasing them all out of the field).

I think the important bit in all these discussions is that "universal health care" and "single payer" aren't synonyms. Single payer is one way to do universal, and most universal systems don't do it. But this article conflates the two.