Alternatively, face masks are this much more effective at blocking much-larger virus particles.

To make such a conclusion based on this research is a logical fallacy.

I’m curious, what would the mechanism be for this to not be true?

Well, they only measured the CO2 content within the mask. The mechanisms were not studied, so you can't make conclusions, and thus you are unable to say whether it is or is not true. (i.e. you can't say anything about filtering capability, it's not what they studied) All they found was a correlation between mask usage and CO2 concentration that exceeds the government safety levely by several multiples within a few minutes. I suspect this has to do with the rate of transmission, but again I cannot make any definitive statements without further studies.