It certainly ignores a lot of human productivity studies that indicate people rarely work at full manual labor productivity past 8 hours and full creative work productivity (including problem solving and often socialization skills) past 4 hours.

It certainly may be the case that they actually are interested in seeing interviewees past such points in a somewhat controlled environment.

Such "gauntlet" interviews seem common among large technology companies and while I think some of it is a semi-intentional attempt to push interviewers to limits to see how they react, I think it's also just as much "everybody else does it" and that continual momentum of existing interview processes. Which is precisely how such hazing processes in organizations get normalized and their problems ignored over the long term. (Yes, pushing someone to the limits of their productivity is strenuous activity and the very definition of hazing.)