I suppose it depends on how you measure "hard".
If you mean "hard" as in "put in my own personal maximum effort", you've got a point. A five year old girl can "work as hard" as a 35 year old man, if they're both trying their best. Hell, a five year old girl can work even *harder* than a 35 year old man, if he's just slacking.
If you mean "hard" as in "actually performed an objectively measurable feat of strength", then, yes, there are some inherent sexual differences, and you can clearly see this in the over-representation of men in objectively hard, dangerous, physical jobs. Your "hard working" five year old girl might be putting 100% maximum effort to lift that 10 pound bag, and the "slacking" 35 year old man might only be putting in 10% effort moving around a 40 pound bag, but the 35 year old man is doing harder work.
I only point this out because GP didn't use the word "effort", which you seem to have interpreted into their comment.
In my experience, there is a significant difference in productivity for men and women, across quite a number of professions. Claiming that there is no difference in the productivity is quite misandrinistic. It's also false.