That is common on construction sites. Our plant had 2000 people working to build it. When done, only about 78 people manned it.
Same for a pipe line, same for an interstate highway, same for building your home. Once done, they are no longer needed and go on to the next project.
Those numbers are for operations, not construction. The pipeline was finished decades ago. That is the number the pipeline company employees today.
At the peak, there was about 30,000 people working on the Alaska Pipeline Construction.
In all, more than 70,000 individuals worked on TAPS’ construction. {Trans-Alaska Pipeline System}
http://www.alyeska-pipe.com/TAPS/HistoryDesignConstruction