UHC proponents are just excessively optimistic about impact on health outcomes.

No, they are excessively optimistic about the financial outcomes. Healthcare should not bankrupt people.

This study appears to just take observational mortality averages between insured and uninsured and then assume that UHC would close the gap. Patently absurd imo, but I guess I'm not a healthcare economist.

They're excessively optimistic (as in, unrealistic) about both. (Healthcare mostly doesn't bankrupt people -- the vast majority are insured, and the rest pay ~nothing.)