I think that Piketty's book is valuable as a warning of the diminishing power of labor vs. people with large amounts of capital. You can nitpick about some of the data and models, but the overall message is right on.
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.
I do think that many mainstream economists don't like what Piketty says because of his criticisms of economists who have economic incentives that color their views and my first reaction was that this paper reflected this status quo loving point of view.