The slower rate of TFP growth occurred before the concentration of wealth, not after. So it's hard to argue that it is related at all to the phenomenon under discussion.
Why can't it be related? I see plenty of ways for the TFP to be related to the concentration.
For example, here's one model which nicely matches the TFP lack of growth and subsequent wealth concentration: 'the wealthy 1% demands a fixed additional sum per year (due to some sort of hedonic treadmill or cultural imperative - relative status seeking perhaps); while TFP growth exceeds this fixed sum, we will see the additional wealth distributed to the other 99% through wages etc. Once TFP falls below this sum, all TFP gains now go to the 1% and everyone else stagnates.'
