How is arguing an empirical or theoretical point "off putting"? You say you don't know the paper well enough to criticize it, but I suggest you read the introduction. It is not technical at all and I'm not sure how you could come away from reading that fairly straightforward explanation of the paper's intention still thinking that it is "off putting".

The overall message of Piketty's book is that capital's share of income will rise due to the difference between capital returns and growth. It may be true that capital accumulation is and will continue to be driving inequality, and is certainly valuable to know if so, but you can't accept the assumptions on which that argument is based blindly.

This paper is not nitpicking a small point, but arguing against the main message of the book.