As for your objection and concern - the study deals with that issue by letting participants decide themselves, what counts as high quality and low quality.
This holds if you look at outright conspiracy theories. Globally, conservative users are the most susceptible to such campaigns.
I will add "at this moment in time". I expect that sufficiently virulent disinfo which targets the left will evolve eventually.
For additional reading, not directly related to lib / con disinfo efficacy - The spreading of misinformation online. https://www.pnas.org/doi/10.1073/pnas.1517441113
This is one of the first papers on this topic I ever read, and will help in the consideration of misinfo / disinfo traffic patterns in a network.
uhh - not that you asked for additional reading.