How does this refute OP's point? They're really just pointing out how absurd it is that a study on wealth inequality and class fails to even consider the Marxist perspective. Genuine scholarly negligence.
> This is also bizarre. Piketty does have a theory about "why inequality is so rampant in the first place". He just thinks we can change the laws and fix it!
OP isn't arguing that Piketty has no theory -- just that it's a pretty naive one. A Marxist perspective understands that "changing the laws to fix it" is not enough. The state is dependent on the bourgeoisie for its survival (see [0]), and will thus bend to their demands. So long as we have no organized and militant labor movements (and labor parties to represent them), the working class will have neither leverage over the state nor the means to enact their demands.
[0]: https://en.wikipedia.org/wiki/Marx%27s_theory_of_the_state#T...