Your statement is false and unsourced. The basis of the paper, as stated in the abstract is to "examine the role of search engines and their optimization processes in directing traffic towards junk news & disinformation about COVID-19" - it has nothing to do with "radical leftist propaganda".

Also, besides Snopes, the authors list the following sources: AFP Factuel, BBC Reality Check, Correctiv, Les Décodeurs, dpa Faktencheck, FactCheck, Media Bias/Fact Check, Newsguard, Pagella Politica, PolitiFact, Tagesschau Faktenfinder.

It is easy, especially for the HN community, to see that overly-sensationalist fake news stories could trip an algo whose purpose is to surface the "most popular" stories.

This is fantastic work by The Computational Propaganda Project.