Those numbers are for operations, not construction. The pipeline was finished decades ago. That is the number the pipeline company employees today.
Given that this company has more than one pipeline, some of the overhead of the Alaskan Pipeline would be spread across multiple pipelines (not new hires), that said, 250 to 300 is very much a lowball. I would expect direct contracts would employ more than that for just trimming trees and killing vegetation along the right of way.