When that paper was written, "Google" referred to an academic research project, not a company. Page and Brin indicate that because other search engines were developed at corporations, they were "largely a black art" and advertising oriented.
Since Google has become a company, and is no longer a research project in the academic realm, it has become a black art and advertising oriented, just as Page and Brin said happened to search engines developed at corporations.
When that paper was written, "Google" referred to an academic research project, not a company. Page and Brin indicate that because other search engines were developed at corporations, they were "largely a black art" and advertising oriented.
Since Google has become a company, and is no longer a research project in the academic realm, it has become a black art and advertising oriented, just as Page and Brin said happened to search engines developed at corporations.