I think they mixed up their sources. The jeep/ox comparison appears to be from the 1944 US Navy report: https://archive.org/details/APracticalExperimentInSimpli

I think they mixed up their sources. The jeep/ox comparison appears to be from the 1944 US Navy report: https://archive.org/details/APracticalExperimentInSimplifiedKeyboardRetraining/page/n24/mode/1up https://archive.org/details/APracticalExperimentInSimplified... (Typewriting Behavior, for the curious: https://archive.org/details/in.ernet.dli.2015.74878/page/n11/mode/2up https://archive.org/details/in.ernet.dli.2015.74878/page/n11...)