It is a good question. One, the book was published relatively recently, but, two, the research and and testing took an inordinately long time to collect. Seems like nielsen could have benefited from SaaS based gaze tracking.
YouEye offers gaze tracks over videos of individual user tests as well as aggregated heatmaps on each page visited by a group of testers with the same task. Gazehawk offers a single heatmap of one static image. There is no user testing with Gazehawk.