>Isn't the original PageRank (...) defined by what Google engineers believe to be a relevant and informative webpage?No, quite the opposite.
The original PageRank was defined by what other internet publishers considered to be a relevant and informative webpage. Specifically, the original PageRank was weighting content by the links (and content around the links) it was getting. The trust was placed on the internet publishers linking to it. The trust was distributed and also transitive to a degree.
Which, if you think about it, is pretty close to what journalists do - they prop up the good ones in various ways. And also reasonably close to the peer review process popular in sciences - equal peers judging relevant materials and marking / linking the ones they approve.
All in all, the original algorithm was effectively a "wall" between Google engineering and the actual content, as ranked & searched. It was clear delineation that "we don't editorialize search results".
Granted, the later incarnations of Google Search take into account much more than just the original PageRank, and thus stray away from the idealized original formulation - and also get much closer to editorial decisions.