Picketty's re-response[1]. It gets rather technical from there on. It's not my area of expertise, either, so I won't come with my opinion on the matter. Acemoglu is a giant, and so is Picketty, so I watch the fight from afar.
The first two levels of discussion (C21 and Acemoglu's immediate answer) are digestible, though.
What's important to understand is that this is all part of a larger discussion on inequality, which has gained a lot of momentum in the last few years in economics (in all fronts -- wealth, income, education, etc.) It's fine to take the position of "I'm not sure" for the moment being, while informing yourself.