He certainly tries to do so, and when he has solid data he does look at returns net of depreciation. It is a clear observation that the type of capital got transformed, however many wealthy people managed to ride the change -many of them were quite diversified and thus able to absorb the change.

But when data is missing, I do get the impression that his proxies tend to be at least somewhat gross of depreciation or maintenance.
In any case, over the 19th century, his conclusions certainly hold. Even if (a big if. He has done his homework and I may be wrong) he slightly overstates the return on capital, it was clear even then that capital owners were diverging from the rest of the population. Clearly, there was a strong accumulative effect. So the depreciation effect was dominated by the return, by some distance.

This was true of much of the 20th century also -but, crucially, there is this period in the middle that made us think otherwise.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi