You can quibble with the specific projected savings, or the projected lives saved. But it kinda doesn't matter? If we can have net savings of half what this paper said, or a quarter, and still have more lives saved, that would still be a strong argument to do it.
Somewhere there's a pareto-optimal frontier where one can't possibly save more lives without spending more, or spend less without more people having negative health outcomes ... and the question is whether universal health coverage would be a step towards that frontier (b/c we're all pretty sure we're far from the frontier today). And the fact that plenty of countries have both lower costs and better outcomes through such a system is highly suggestive that it is a more efficient policy regime.
Maybe the big quibble is whether the American penchant for creating corporate givewaways to powerful organizations that lobby politicians would create an especially toxic public-private-partnership monopoly in which the biggest existing private healthcare networks are granted regional monopolies and set crazy prices (or some other dystopian warping of an initially reasonable idea) so costs actually balloon. I do think that would be a risk and so we need to be careful about the specifics of how we implement this.