They're probably one of the worst examples. It was technical brilliance, not their idea, that allowed them to succeed.
EDIT: At this point their brand is also a source of Google's money. Today you need both to compete in search.
[1] http://www.av-comparatives.org/comparativesreviews/main-test...
My take on this is.
If your idea is good you might as well put it out there as you won't be the only one who have thought about it.
If your idea is brilliant it will have no takers as brilliant ideas seem to be less obvious in the now and will only with time and in retrospect be obvious.
If PageRank was closely guarded in any way, it could have been so that Page and Brin could submit their research paper about it to SIGIR--at which point it would be public. On the other hand, Page and Brin hosted the search engine for anybody to use.
2) As an aside, assuming the page below is the actual paper, does it mean Larry and Brin already have funding at the time they were writing it up? It mentions that users should try the search engine out at google.stanford.edu, but supposedly, it was their first investor who named Google.