It's not really an adequate comparison. I work in orthopedics and rehabilitation, and modern people do indeed acquire specific chronic orthopedic ailments based on their occupation.
Most of these injuries are acquired from jobs where you repeat specific motions all day. It doesn't really mean you've done hard labour, more that you've over used specific muscle groups and joints.
Btw I do agree with your general rebuttal, that any work back then was much more labour intensive. I just don't know if that particular anthropological fact lends much weight to your argument.
You'd probably get better information examining the average age of the working male. From anecdotal experience, hard labour is a young mans game. I work in oil country, and I don't ever have any old rough necks as patients. At least not one's whole are still working.
I'm not at home in this field. I have looked at Non-Slave Labour in the Greco-Roman World by Garnsey, and can probably hop on from there, but would you mind providing more details on the sources, e. g. are you referring to the economist Richard D. Wolff? Any particular papers / DOIs you could provide?
It depends on when in history you are comparing from. For most of human history, humans as hunter gatherers worked on average only 3-8 hours each day.
Agrarian societies worked similar number of days each year, but work was heavily dependent on weather and seasons. It was the sudden shift to proto industrialisation and industrialisation that brought about an extreme increase to 60-80 hour work weeks, but in the spam of human history this is a very small minority.
I think that depends on what kind of slave you were..... Debt slavery, yeah not the worst thing that could happen. Penal slavery, or slave of war........? No thank you. Not much is really comparable to the fate of being a penal slave mining silver in Iberia. It was a death sentence carried out over a period of being worked to death while breaking rocks.