Researchers want the public to test themselves: https://yourmist.streamlit.app/. Selecting true or false against 20 headlines gives the user a set of scores and a “resilience” ranking that compares them to the wider U.S. population. It takes less than two minutes to complete.
The paper
Edit: the article might be misrepresenting the study and its findings, so it’s worth checking the paper itself. (See @realChem 's comment in the thread).
Rounding error :)
100% has a margin of error in millions, or tens of, depending on interpretation.
For most purposes, a 5% margin of error is considered acceptable. Since some quick websearch estimates 336M people, if up to ~17M people are informed, you can still claim that 100% is misinformed.
(But then, if you actually know how many people are informed, your acceptable margin of error falls down considerably.)
I chose to believe Koreth is not actually a member of the US population, but a geographically confused Canadian.