The credibility pyramid comes from a preponderance of papers, and any flaws in early research are masked by quoting their conclusions in later papers as supporting 'evidence'.
Without going back to that paper, you can't assess the validity of those conclusions.
I followed a similar pyramid in digging through about 40 years of motorcycle safety studies, which used the same M.O., cross referencing conclusions which in many instances were unsupported by data.
The trail went cold when I tried to find a paper done on helmet use in military motorcycle dispatch riders from 1947--I never did get that one, but it was the cornerstone of the other early studies I went over.
Interesting observation, food for thought in many areas.