aggregate return on capital (i.e. the sum of profits of all firms i.e. all the money that rich people make) is completely different than the return of any one firm or market/investment strategy. you seem to be conflating the two deliberately to set up a straw man.
I agree with your response to your parent post, but I have one clarification to add:
aggregate return on capital (i.e. the sum of profits of all firms i.e. all the money that rich people make)
Actually, as I understand it, the term "capital" as standardly used by economists is broader than this. To see the difference, consider: if you are doing work that requires skills (as I'm sure most HN readers are, since computer programming is such work), those skills are capital that you possess, and your pay, according to standard economic usage, is partly return on that capital, not compensation for labor. (Basically, the difference between your pay and the pay of an unskilled person, like the burger flipper at McDonald's, is return on capital--it's actually not quite that simple because variations in productivity at similar skill levels, due to automation, also come into play, but that's the gist of it.)
I have not read Piketty's book, so I can't say for sure how he is using the term "return on capital", but it seems to me that in order to properly interpret the data he appears to be using, one has to pay very careful attention to the distinction I made above. Otherwise one might be incorrectly estimating return on capital.
