While interesting, this hypothesis of why it took so long for humans to populate North America is still pretty tenuous. Giant killer hyenas as the primary roadblock to western hemispheric expansion definitely has problems. One of the hallmarks of early-modern human occupations of any region is predators simply evaporate.

This is most starkly represented in the Levant (Israel/Lebanon/Syria/Jordan), which was occupied by Neandertals very early. During Neandertal occupation, you had very clear stratigraphy indicating the Neandertals were seasonally sharing living space with the other big predators of the region. As soon as you get signs of early modern humans both the Neandertals and the other big predators just disappear, and there's no seasonal sharing anywhere. And there were predators every bit as nasty roaming the savannah and beyond (cave lions, 6-foot-tall baboons, cave bears, etc.) as there were in the North American region.