Looks like that was published in 1998, but that's also the year they submitted their patent application for PageRank. So you couldn't use it because they didn't want to risk losing their budding search business to, say, Microsoft.

In later years Google generally published the details of a system after they moved to the next one, to stay ahead of their competition, or if they didn't see it as important to their business (most infamously the Transformer language model). It was still generous of them to share their research at all though, and this allowed their scientists to talk publicly about their research which is a huge plus.